Green’s Dictionary of Slang

a fool, a simpleton.

c.1510		Colyn Blowbols Testament line 371: Sybour Groutehed a man full discrette, Whiche wil be dronke with myghti wynes swete.
1570		Levens Manipulus vocabulorum n.p.: An assehead [...] A blockhead [...] a grouthead.
1578		Florio Firste fruites 58: [T]here were, are, and I beleeue, wyl be certaine groutheaded, yl manered, and to say better, enuious [people].
1593		‘Philip Foulface’ Bacchus’ Bountie in Harleian Misc. II (1809) 307: His name was Gotfrey Grouthead; and with him he brought a wallet full of woodcocks’ heads.
1599		Nashe Praise of the Red Herring 39: Those Turbanto grout-heads, that hang all men by the throates on Iron hookes.
1630		J. Taylor ‘Sir Gregory Nonsense’ in Works (1869) II 3: There you may see many a greedy grout-head, / Without wit, or sence, almost without-head.
1677		G. Miege A new dictionary French and English n.p.: GROUTHEAD, tête de beuf.
1697		T. Oates Eikōn vasilikē tetartē 57: [O]ld Grouthead, your Scotch Favourite, he advis’d [...] that the Plot it self should be made use of as a Cripple to beg Mony .
1841		Staffs Gaz. 20 May 4/6: Yet trust not Hob Grouthead for sleeping too long.