"sentimentally woebegone" [Century Dictionary], 1768, lack-adaysical (Sterne), from interjection lackadaisy "alas, alack" (1748), a ludicrous alteration of lack-a-day (1690s), an exclamation of sorrow or regret, from alack the day (1590s). Hence, "given to crying 'lack-a-day,' vapidly sentimental." Sense probably altered by influence of lax. Related: Lackadaisically.
updated on April 25, 2016