Alexander Dyce, A General Glossary to Shakespeare's Works

speak “thick,” CYMBELINE, iii. 2. 55 ; “speaking thick,” 2 HENRY IV., ii. 3. 24. “Speaking thick is speaking fast, crowding one word on another” (STEEVENS) ; “without proper intervals of articulation,” Johnson's Dict., sub “thick” (In Chapman's Commentary on the Iliad, B. iii. we find “ἐπιτροχάδην, signifying velociter, properly modo eorum qui currunt; he spake fast or thicke.” p. 48 ;— “which agreeth not the lesse with his fast or thicke speaking.” p. 49, ed. folio ); and see thick.