The red, bloodshot face and/or eyeballs caused by violently vomiting.
Chunder strain is caused by the violent muscle spasms bursting the blood vessels under the skin and in the eyeball creating a red, blotchy face similar to that of an allergic reaction.
Guy 1: Jesus, you look like you've been beaten up or on pills!
Guy 2: Nah I just had too much red wine and was sick last night.
Guy 1: That's a bad case of chunder strain!