Officially, imprimo means I mark/stamp (hence English impression), where premo just means I press.
The nuances of Latin prefixes have long fascinated me, and it took me forever to realize that they’re basically equivalent to English verbs that contain prepositions. So here it‘s a case of to press vs. to press in. Other examples are:
- rideo = I laugh; inrideo = I laugh at.
- lumino = I shine; illumino = I shine onto [something].
It gets a little more complicated when there aren‘t English equivalents:
- lacrimo = I cry; illacrimo = I burst into tears.
- amplexor = I hug [someone]; complexor = I give [someone] a hug.
But you can usually figure it out with some thought. Nevertheless, it seems very strange to me that this principle isn’t taught as part of standard Latin instruction. When you think about it this way, it’s not difficult, but I’ve never seen it explained anywhere.
Wait, actually, that’s not true. In fact, Alfonso Traina, Professor Emeritus at the University of Bologna, has written about this, but unfortunately 1) he writes in Italian and 2) I only have photocopies of a few sections of his work, and they don’t contain the title of the books they’re from, so I’m unable to point you to specific references. :(
Some example sentences:
Nam et vipera et caecilia saepe cum in pascuo bos improvide supercubuit, lacessita onere morsum imprimit. (Columella, Res Rustica, 6.17.1)
For the cow when in the field often lies down on the viper and the lizard; provoked by its weight, [the viper or the lizard] impresses its bite [on the cow].
Simulque alteram fasciam minister attollit, alteram premit medicus. (Celsus, De Medicina, 8.10.2b)
And at the same time the assistant lifts one loop up while the doctor presses on the other. (In a discussion about setting broken bones.)