Etymology Dictionary

QUAFF,  to drink in large draughts.  (C.)   In Shak. Tw. Nt. i. 3. 14; &c.   And in Levins.  The double f stands for a guttural.   The true form is quach (ch as in German), meaning to drink out of a quach or cup, called quaich, quech, or queff in Lowland Scotch; see quaich in Jamieson.   'I quaught, I drink all out;' Palsgrave.   Thus to quaff is to cup; 'Cup us till the world go round;' Antony, ii. 7. 124.—Irish and Gael. cuach, a cup, bowl, milking-pail.   Cf. W. cwch, a round concavity, hive, crown of a hat, boat.   Perhaps from ✔KU, to contain; see Cave.  Der. quaff-er. [†]

ADDENDA

I regard the final -t in Palsgrave's quaught as due to a sb. quaught, a draught, in which the -t is suffixed, as in draugh-t from draw, laugh-t-er from laugh; cf. also hois-t, waf-t, graf-t.   G. Douglas has waucht, to quaff (see Jamieson), but Dunbar has the simple form, as in:  'They wauchit at the wicht wyne,' they quaffed at the strong wine; Maitland Poems, p. 46.   This is decisive as to the later addition of t.   Cf. 'The queff, or cup, is filled to the brim;' Hone, Tablebook, i. 467.