C17: from Latin stolidus dull; compare Latin stultus stupid; see still1
Word Origin and History for stolid
1560s (implied in stolidity), from Middle French stolide (16c.), from Latin stolidus "insensible, dull, brutish," properly "unmovable," related to stultus "foolish," from PIE root *stel- "to put, stand" (see stall (n.1)).