Green’s Dictionary of Slang

sap-headed adj.

foolish.

1665		Head Eng. Rogue I 48: Culle, A Sap-headed Fellow.
1691		N. Ward ‘The Poet’s Ramble after Riches’ in Writings (1704) 14: You Rogue, you Rascal, are you not, / A Silly, Sorry, Sap-Head Sot.
1816		Exeter Flying Post 29 Aug. 2/1: Some sap-headed waterman got too many in his boat, and it went over.
1821		Dublin Wkly Register 8 Dec. 4/2: Sap-headed fools!
1831		Cobett’s Wkly Polit. Register 29 Oct. 17/1: Here are the Carters [...] amongst the old sap-headed Whigs.
1836		T. Haliburton Clockmaker (1843) I 30: Sap-headed enough too for refuse, said a stout-looking farmer.
1848		Boston Blade 17 June n.p.: I saw curls, the sap-headed waiter at the American House.
1854		Life in Boston & N.Y. (Boston, MA) 13 May n.p.: Bah! thou poor sap-headed nincompoop.
1887		B. Harte Crusade of Excelsior Pt II Ch. 1: These sap-headed fools don’t, of course, know THAT.
1893		Aberdeen Jrnl 2 Aug. 3/1: Well, you are a sap-headed idiot.
1902		Harper’s Mag. Jan. 266/2: Of all the sap-headed milksops.
1904		Sun. Times (Perth) 4 Dec. 1/1: The sapheaded ex-swanky seller’s arrogant assumptions of superiority excite Foulkes to frenzy.
1914		Truth (Melbourne) 10 Jan. 11/6: The sapheaded youth they hook with the tale.
1916		E. Pound letter 9 Mar. in Paige (1971) 71: I think from a note of Mathews’ which I have mislaid that he, like the sap-headed imbecile that he is, has sent your lot of books to the infernal chasm of the Boston Transcript.
1923		Wodehouse Inimitable Jeeves 177: A perfect sapheaded prune like young Bingo.
1929		E. Wheelan Don. K. Haughty [comic strip] And so the dippy don and his saphead servant fared forth.
1945		Chelmsford Chron. (Essex) 19 Jan. 2/3: He was [...] never discouraged with the sap-headed coadjutors who [...] block the way of all efforts of improvement.

In derivatives

sapheadism (n.)

foolishness.

1909

J. Ware Passing Eng. of the Victorian Era 215/1: Sapheadism (Agricultural, Amer.). When the sap is rising, the bark is soft-hence this term for weak-headedness.