Sap

Meaning a fool or simpleton is from the early 19th century and is short for the older and now obsolete expressions ‘sap-skull’ or ‘sap-head’, which date from a century earlier. Sap, of course, is the lifeblood or juice that circulates in plants and trees and this word has been around since Anglo-Saxon times. From trees, of course, is the connection to wood or timber, which has long been associated with stupidity as in 'blockhead' or 'wooden-headed'. From this, it is but a short step to 'thick as a plank' or, for even more emphasis, 'thick as two short planks'. 'To sap one's strength' is related to the lifeblood of trees, meaning to diminish or weaken one's strength, and this usage dates from the mid-1700s. 'To sap' meaning to dig a trench or tunnel dates from the late 1500s. See also Sapper.