As nouns the difference between coster and cosier
is that coster
is costermonger while cosier
is (obsolete) an incompetent tailor.
As an adjective cosier is
The Mirror and the Lamp
, passage=The turmoil went on—no rest, no peace. […] It was nearly eleven o'clock now, and he strolled out again. In the little fair created by the costers'
barrows the evening only seemed beginning; and the naphtha flares made one's eyes ache, the men's voices grated harshly, and the girls' faces saddened one.}}
Compare (etyl) coussier maker of mattresses, or couseor tailor.