Similar to phrases like oh my god or jesus christ, fuck-in-a-bucket is meant to be exlamatory or an adjective to describe someone particularly insane or eccentric.
(1)
Dude346: LoL wot did u an lexy do 4 ur dayte?
Guy499: Omg, we fucked in a box
Dude346: LOL *fap* *fap* *fap*
(2)
Guy 1: Dude...I'm sorry. I got drunk and fucked your wife.
Guy 2: FUCK IN A BOX!
(Phrase) Used to express extreme disbelief. Usually used after witnessing something exceedingly stupid, or being tasked with something exceedingly difficult. Sometimes also used to express frustration.