monosyllable (n.)

"a word of one syllable," 1530s, from Latin monosyllabus "of one syllable," from Greek monosyllabos, from monos "single, alone" (from PIE root *men- (4) "small, isolated") + syllabē "syllable" (see syllable). That the oldest words in English tended to be monosyllables was noted from 16c.