The Answer
Afterhand actually was a word that dates back to the 15th century and is the opposite of beforehand. For an unknown reason, one word survived and the other didn’t.
Here’s a sentence we might say – “If we’re going to the event, let’s meet for dinner beforehand.” However, we’re unlikely to say “If we’re going to the event, let’s go out for drinks afterhand.”
It turns out that there was a time when both would be equally valid.
We talk about something occurring beforehand, so why don’t we talk about something happening afterhand? Actually, afterhand goes all the way back to 15th-century English, even though it’s not that commonly used today.
According to the Oxford dictionary, afterhand’s origin is “Late Middle English. From after + hand.” and the definition is “Afterwards, subsequently; after the event.“
I was unable to find a reason for why beforehand remained common in modern English, and afterhand did not. With a little effort, I think we can bring it back. Who’s with me? We can use the word a lot and then go out for drinks afterhand.
Why hand?
I’ve been unable to find a satisfactory explanation for what the hand part of before/afterhand means. In the Oxford dictionary origin of beforehand, it says “Middle English (originally as two words): from before+ hand; probably influenced by Old French avant main.“
Okay, great. What does avant main mean?
Partie antérieure des animaux, et en particulier du cheval, comprenant la tête, l’encolure, le garrot, le poitrail et les membres antérieurs.
Help me Google Translate – you’re my only hope. This means – “Anterior part of the animals, and in particular the horse, including the head, the neck, the withers, the chest and the forelegs.“
So, it means the front of a horse. I suppose the front of a horse arrive before the back of a horse, so if you arrive beforehand, you’re getting there in advance the way the front of a horse does. They must have had very long horses in France long ago.
There is a conversation on Reddit where this is the subject of debate. The first answer suggests that the first edition of the Oxford English Dictionary gives the following reason.
Originally two words, before hand, also before the hand, perhaps from the idea of one working before the hand of another, and so in anticipation of his action. But cf. Latin prae manu, manibus, ‘at hand, in readiness, in hand,’ used in ME as = “beforehand”.
The link it provides as a reference is dead, so I’m unable to confirm. So, I’m going to stick with horse-front.