"speaking or using only one language," by 1939, from mono- "single, alone" + ending from bilingual, etc.
Entries linking to monolingual
mono-
word-forming element of Greek origin meaning "one, single, alone; containing one (atom, etc.)," from Greek monos "single, alone," from PIE root *men- (4) "small, isolated."
bilingual adj.
1818, "speaking two languages;" 1825, "expressed in two languages;" see bi- "two" + lingual. Latin bilinguis meant literally "two-tongued," and, figuratively, "speaking a jumble of languages," also "double-tongued, hypocritical, false."