FILLOCK

also FILLOK, FYLLOK

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from E-NED: 
of obscure formation; the original sense is perhaps ‘filly’ (cf. Welsh ffilawg filly, wanton girl, in Salesbury 1547 filoc, which may be an early 
adoption from English). 
The word may be from filly + -ock, or represent (with Northern pronunciation) 
an Old English *fylece wk. fem., corresponding to Old High German fulihha:
—Old Teutonic *fulikôn-, f. *ful-, fol-

? c 1450 - Hoccleve's The Letter of Cupid to Lovers
see below From: Hoccleve's Works
The Minor Poems
Published by the Early English Text Society 1892
XIX. The Letter of Cupid to Lovers, His Subjects
P. 38

From: The Literature of Roguery
By Frank Wadleigh Chandler
Volume I. 1907
Chapter III. The Anatomies of Roguery. I. Beggar Books
P. 88