Arabic Root:
From the root shin-ha-dal (ش ه د), which has the following classical Arabic connotations: to bear witness, to offer testimony, to have knowledge of, to experience, to be present with.

An Attribute of Ilm (knowledge):
To be a witness is an attribute of knowledge. You cannot be a witness without being present or having knowledge or so unique insight or perspective on some matter. Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ is Ash-Shaheed, THE witness. He is the knower of the Ghayb (the unseen), the things that are concealed or invisible to man. It is because of the attribute of being Ash-Shaheed that on the day of judgment, He will reveal Himself as being the witness to all that has happened.

يَعۡتَذِرُونَ إِلَيۡكُمۡ إِذَا رَجَعۡتُمۡ إِلَيۡهِمۡۚ قُل لَّا تَعۡتَذِرُواْ لَن نُّؤۡمِنَ لَكُمۡ قَدۡ نَبَّأَنَا ٱللَّهُ مِنۡ أَخۡبَارِكُمۡۚ وَسَيَرَى ٱللَّهُ عَمَلَكُمۡ وَرَسُولُهُۥ ثُمَّ تُرَدُّونَ إِلَىٰ عَٰلِمِ ٱلۡغَيۡبِ وَٱلشَّهَٰدَةِ فَيُنَبِّئُكُم بِمَا كُنتُمۡ تَعۡمَلُونَ,

Ya'taziroona ilaikum izaa raja'tum ilaihim; qul laa ta'taziroo lan nu'mina lakum qad nabba annal laahu min akhbaarikum; wa sa yaral laahu 'amalakum wa Rasooluhoo thumma turaddoona ilaa 'Aalimil Ghaibi washshahaadati fa yunabbi'ukum bimaa kuntum ta'maloon

English Translation:
"They will make excuses to you when you have returned to them. Say, 'Make no excuse – never will we believe you. Allah has already informed us of your news. And Allah will observe your deeds, and [so will] His Messenger; then you will be taken back to the Knower of the unseen and the witnessed, and He will inform you of what you used to do.'"— (Qur'an 9:94)

ٱرۡجِعُوٓاْ إِلَىٰٓ أَبِيكُمۡ فَقُولُواْ يَـٰٓأَبَانَآ إِنَّ ٱبۡنَكَ سَرَقَ وَمَا شَهِدۡنَآ إِلَّا بِمَا عَلِمۡنَا وَمَا كُنَّا لِلۡغَيۡبِ حَٰفِظِينَ

Irji'ooo ilaaa abeekum faqooloo yaaa abaanaaa innab naka saraq; wa maa shahidnaaa illaa bimaa 'alimnaa wa maa kunnaa lilghaibi haafizeen

English Translation:
"Return to your father and say, "O our father, indeed your son has stolen, and we did not testify except to what we knew. And we were not witnesses of the unseen,"— (Qur'an 12:81)

This verse also helps us establish a connection to the previous name, Al-Baith - The Resurrector. When we are raised from the grave, we will be held accountable. The only one who can be a judge is the one who has knowledge of the truth, "we did not testify except to what we knew." Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ knowledge is all-encompassing. He is Al-Alim (The All-Knowing) and Al-Khabir (The All-Aware). The explanation of the name Ash-Shaheed is similar to that of those names. We recommend reading the write-up provided for those names to become acquainted with the name of Ash-Shaheed.

Now, as a name of Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ it is mentioned throughout the Qur'an a total of 18 times. We will explore a few more verses here:

يَوۡمَ يَبۡعَثُهُمُ ٱللَّهُ جَمِيعٗا فَيُنَبِّئُهُم بِمَا عَمِلُوٓاْۚ أَحۡصَىٰهُ ٱللَّهُ وَنَسُوهُۚ وَٱللَّهُ عَلَىٰ كُلِّ شَيۡءٖ شَهِيدٌ

Yawma yab'asuhumul laahu jamee'an fayunabbi'uhum bimaa 'amiloo; ahsaahul laahu wa nasooh; wallaahu 'alaa kulli shai'in shaheed

English Translation:
"On the Day when Allah will resurrect them all and inform them of what they did. Allah had enumerated it, while they forgot it; and Allah is, over all things, Witness."— (Qur'an 58:6)

مَّآ أَصَابَكَ مِنۡ حَسَنَةٖ فَمِنَ ٱللَّهِۖ وَمَآ أَصَابَكَ مِن سَيِّئَةٖ فَمِن نَّفۡسِكَۚ وَأَرۡسَلۡنَٰكَ لِلنَّاسِ رَسُولٗاۚ وَكَفَىٰ بِٱللَّهِ شَهِيدٗا

Maaa asaabaka min hasanatin faminal laahi wa maaa asaaabaka min saiyi'atin famin nafsik; wa arsalnaaka linnaasi Rasoolaa; wa kafaa billaahi Shaheedaa

English Translation:
"What comes to you of good is from Allah, but what comes to you of evil, [O man], is from yourself. And We have sent you, [O Muhammad], to the people as a messenger, and sufficient is Allah as Witness."— (Qur'an 4:79)

سَنُرِيهِمۡ ءَايَٰتِنَا فِي ٱلۡأٓفَاقِ وَفِيٓ أَنفُسِهِمۡ حَتَّىٰ يَتَبَيَّنَ لَهُمۡ أَنَّهُ ٱلۡحَقُّۗ أَوَلَمۡ يَكۡفِ بِرَبِّكَ أَنَّهُۥ عَلَىٰ كُلِّ شَيۡءٖ شَهِيدٌ

Sanureehim Aayaatinaa fil aafaaqi wa feee anfusihim hattaa yatabaiyana lahum annahul haqq; awa lam yakfi bi Rabbika annahoo 'alaa kulli shai-in Shaheed

English Translation:
"We will show them Our signs in the horizons and within themselves until it becomes clear to them that it is the truth. But is it not sufficient concerning your Lord that He is, over all things, a Witness?"— (Qur'an 41:53)

Tip to help memorize the meaning of this name:
To help remember the meaning of Ash-Shaheed, it can be useful to make the connection in your mind with this name and the shahadah (the testimony of faith), Ashhadu an la ilaha ill-Allah was ashhadu anna Muhammadan abduhu wa rasuluh (I bear witness that there is none worthy of worship except Allah, and I bear witness that Muhammad is his servant and Messenger).^[1] Here ashhadu means, "I bear witness," "I testify" and the name of Allah Ash-Shaheed is, "The Witnesser," "The Testifier."

Reflection:
The believer benefits from knowing Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ is Ash-Shaheed. As we mentioned in previous names, it makes us cognizant of our actions. We will be held responsible on the day of judgment, so unattractive qualities within ourselves (e.g., envy, anger, pride, back-biting, etc.) should be conquered as these emotions work against you. This name also encourages humility, admitting mistakes, taking ownership over your actions, and seeking forgiveness from Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ. So on that day, these mistakes will already be wiped from your account.

Remember Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ is enough for you as a witness. Whatever good you do, if no one sees or recognizes it, it doesn't matter as Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ sees all. Nothing goes unaccounted for. Having this knowledge allows a person to drop all forms of pettiness. Who cares if you did something good for someone and it wasn't reciprocated. You invited them to your wedding, but they didn't invite you. Allah is the witness, don't let others' lack of care affect the good you can do.

References:
01) Sahih (Darussalam) Sunan An Nasai (632) and Sunan Ibn Majah (470) | The Messenger of Allah ﷺ in the hadith said: Whoever performs Wudu and does it well, then says the shahadat. Eight gates of Paradise will be opened for him, and he may enter through whichever one he wishes.