Compare quoin, coign, and coin. The brief etymology in my dictionary traces them all back to a word for "wedge" which
goes back to "culex" = gnat (also mosquito)possibly figure of speech about the blood sucking apparatus. The mosquito
has three needles side by side, twp of them capable of being alternately rapidly thrust forward, a very sharp wedge, which pierces tiny blood vessel, injects anticoagulant, and capilarry attraction makes the three closely spaced needles
draw up the blood.
Possibly small wedges of silver were used for money before round coins wer made.And wedges cut from coins were used as money fairly recently "Shave and haircut, two bits!"
Pieces of eight, etc.