Where does “Contra Costa County” come from?

Contra Costa County (English) comes from Spanish contra costa, from Spanish costa, from Galician Costa, from Old Portuguese costa, from Latin costa, from Proto-Indo-European kost-.

Ancestry of “Contra Costa County”, step by step

Step	Language	Word	Meaning
1	Spanish	contra costa	—
2	Spanish	costa	coast, shore, coastline, shoreline; expense,...
3	Galician	Costa	{{m\|gl\|costa\|gloss=slope}}, but also the name of...
4	Old Portuguese	costa	—
5	Latin	costa	a rib; a side, a wall
6	Proto-Indo-European	kost-	—

Every word from Proto-Indo-European kost- →Every word from Latin costa →Every word from Old Portuguese costa →