raincoat

Raincoat is simply rain plus coat, joined in English to name a coat for the rain. Rain traces back through Old English reġn and Proto-Germanic *regną to Proto-Indo-European *Hréǵʰ-der or *Hréǵʰnos, while coat came into English from Old French cotte, from Latin cotta, from Proto-Germanic *kuttô.

There's no recorded shift in meaning here — rain and coat each had their own long history before English simply fused them into a single practical compound.

Coat shares its root with the Latin-derived cotta, so raincoat's second half is a distant cousin of any English word borrowed from that same Old French cotte lineage.