Green’s Dictionary of Slang

groutheaded adj.

foolish, simple.

1589		Nashe Almond for a Parrat 7: It was told me [...] that your grout-headed holinesse had turnd vppe your heeles like a tired iade in the meadow.
1607		T. Walkington Optic Glasse of Humors 64: A man is euer [...] by reason of his sad heauy humor, alwaies stoically visaged, like groutheaded Archesilas.
1612		Passenger of Benvenuto n.p.: Four thousand times you are a grout-headed gull [N].
1639		J. Taylor Crabtree Lectures 95: Thou shalt have the mends in thine owne hand, like a Grout-headed booby as thou art.
1653		Urquhart (trans.) Gargantua and Pantagruel (1927) I Bk I 103: The bunsellers or cake-makers [...] did injure them most outrageously, calling them [...] jobbernol goosecaps, foolish loggerheads, flutch calf-lollies, grouthead gnat-snappers [...] and other such like defamatory epithets.
1696		Motteux (trans.) Gargantua and Pantagruel (1927) II Bk V 557: We are a silly sort of grout-headed lobcocks.