From The Collaborative International Dictionary of English v.0.48 :
Saphead \Sap"head`\, n.
A weak-minded, stupid fellow; a milksop. [Low]
[1913 Webster]
From WordNet (r) 3.0 (2006) :
saphead
n 1: a person who lacks good judgment [syn: fool, sap,
saphead, muggins, tomfool]