Early 16th century; earliest use found in John Fitzherbert (d. 1531). From small + mouthed. With the specific use designating fishes, compare later smallmouth.
Definition of small-mouthed in US English:
small-mouthed
adjective
Having a small mouth.
Origin
Early 16th century; earliest use found in John Fitzherbert (d. 1531). From small + mouthed. With the specific use designating fishes, compare later smallmouth.