From Latin 'collum' (neck), from PIE *kwel- (to turn) — the neck is where the head turns.
The part around the neckline of a garment; a band put around the neck of an animal.
From Old French coler (collar, neck-band), from Latin collāre (band for the neck), from collum (neck), from PIE *kʷel- (to turn, to go around, to revolve) — the neck being literally the turning point that rotates the head. The same PIE root *kʷel- produced Greek kyklos (circle, wheel), Latin colere (to cultivate, to till, literally to go around), and Sanskrit cakra (wheel, circle). A collar is fundamentally a neck-band, something encircling the pivot between head and body. The English
'Collar,' 'accolade,' and 'decollate' all come from Latin 'collum' (neck). A collar goes around the neck. An 'accolade' was originally an embrace around the neck (the ceremony of knighting involved a neck-embrace). To 'decollate' is to un-neck — to behead. And 'blue-collar'/'white-collar' use the collar as a class marker: manual workers wore blue; office workers wore white.