Meaning
A mild exclamation of shock, surprise, or frustration — 'good grief!', 'what on earth!' It is a softened, respectable stand-in slipped in where a much stronger Jamaican curse word would otherwise fall, letting a person vent alarm or annoyance without cursing outright. Reaching for 'backfoot!' rather than something coarser keeps things decent while still registering real surprise, and it belongs to the useful store of minced oaths that stand politely in for the island's forceful profanity.
Examples
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Backfoot! Wah happen yah?
Good grief! What happened here?
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Backfoot, mi lose mi phone again.
Damn, I lost my phone again.
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A wah di backfoot yuh a do?
What on earth are you doing?
Origin
A euphemistic minced oath — using an everyday word ('back foot') to stand in for stronger profanity, the way English uses 'shoot' or 'dang'.
How it’s used
Milder and more acceptable than the '-claat' curse words, though still informal. Safe in most everyday company.