aspect N3 uncommon casual
〜じまう — rough/colloquial variant of 〜でしまう: 飲んじまう
〜じまう ・ じまう
A full written guide for 〜じまう is on the way.
This point is mapped in the grammar tree, but its examples, formation table and usage notes haven't been written yet. The fastest way to learn it now is to meet it in real sentences inside Jengo.
See 〜じまう in real sentences
Jengo shows 〜じまう the way you actually meet it: inside real Japanese sentences, so it sticks instead of staying an abstract rule.
Study it in JengoSources Compiled with reference to A Dictionary of Basic Japanese Grammar. Editorial confidence: medium.