You’d think I’d be familiar with the etymologies of the basic English vocabulary words, but I keep running into surprises. This one comes courtesy of aldiboronti at Wordorigins.org: wanton has the only survival in modern English of a formerly common prefix, wan-, about which the OED says:
a prefix expressing privation or negation (approximately equivalent to UN-1 or MIS-), repr. OE. wan-, won-, corresponding to OFris. wan-, won-, OS. wan- (only in wanskefti misfortune = OE. wansceaft), MLG., MDu. wan- (mod.Du. in many new formations, esp. in the sense ‘wrong’, ‘mis-’, as in wanbestuur misgovernment, wanluid discordant sound), OHG. wan-, wana (only in wanwâfan unarmed, wanaheil unhealthy, infirm, wanawizzi lacking wit, insane), MHG. wan- (only in wanwitze inherited from OHG.), mod.G. wahn- (in wahnwitz, wahnsinn insanity, commonly apprehended as compounds of wahn n., delusion; also in some dialect words, chiefly adopted from LG.); ON., Sw., Da. van- (in many old formations, to which mod.Sw. and Da. have added many more, chiefly adopted from LG.). The prefix is in origin identical with WANE a.
In OE. the number of words formed with the prefix is considerable, but none of them has survived into modern English, and only one (wanspéd, ill-success) into ME. Of the many new formations that arose in ME., only wantoȝen, undisciplined, WANTON, still survives in use (with no consciousness of its etymological meaning)…
And here all these years I just assumed Wahnsinn was from Wahn. This wan- is probably related to Latin vānus ‘empty, idle, vain.’ As for the second part, toȝen is the past participle of téon ‘to discipline, train,’ a strong verb (past téah, tuȝon) related to German ziehen, zog, gezogen, Goth. tiuhan, táuh, tauhum, tauhans, and Latin dūcere ‘to lead, draw,’ as well as to English tow ‘to draw, pull.’ So now you know.