Etymology: < Anglo-Norman foli, foly, Anglo-Norman and Old French, Middle French folie, Middle French follie (French folie ) foolish talk, nonsense (c1100), wrongdoing, sin, morally wrong act (first half of the 12th cent.), madness, insanity, foolishness, foolish action, wantonness, loose living, sexual intercourse (all late 12th cent.), sexual ardour (beginning of the 13th cent. or earlier), wickedness (13th cent. or earlier) < fol foolish, mad (see fool n.¹) + -ie -y suffix³.Compare Old Occitan folia , follia , folhia , Catalan follia (11th cent.), Spanish †folia , Italian follia (both 13th cent.). Specific senses. In sense 5 probably simply a specific sense development; compare post-classical Latin stultitia Huberti , literally ‘Hubert's folly’ (a1236 in Roger of Wendover's Flores Historiarum), the name of a castle constructed at great expense by Hubert de Burgh in the Welsh Marches in 1228. However, perhaps compare also French folie country house (earliest in place names), alteration (by association with fou foolish: see fool n.¹ and adj.) of feuillée , (regional) feuillie foliage collectively, hut covered with leaves (both 12th cent. in Old French; < feuille leaf (see feuille n.) + -ée , suffix forming nouns). In sense 6 probably after French folie comical vaudeville performance (1834 or earlier).

Etymology: Origin uncertain. Perhaps a specific sense development of folly n.¹With use in sense 1 compare earlier use of folly in place names in north-eastern America, especially in Maryland, where it is very frequent as the second element in names of land patents and estates in the 17th and 18th centuries (compare e.g. Richardson's Folly (1662), Elliott's Folly (1685), etc.). Such use may in many cases have arisen with allusion to an owner's perceived foolishness (compare folly n.¹ 1c), e.g. in purchasing land which is difficult or expensive to farm or maintain, although the motivation for its use in individual cases is difficult to determine. Sense 2 may reflect a transferred use of folly n.¹ 5b, reflecting the fact that decorative structures of this kind are often situated on hills with the intention of improving a view; connection with French words relating to foliage discussed in the etymology of folly n.¹, although plausible semantically, seems unlikely given the relatively late appearance of this use in English.

Etymology: Probably partly < folly n.¹, and partly < fool n.¹ + -ly suffix¹. Compare earlier folly adv., and also earlier fool adj. and slightly later foolish adj.

Etymology: Originally < Anglo-Norman foleier, Anglo-Norman and Middle French folier, foloier (French (now regional) foloyer ) to err, to sin (both first half of the 12th cent. in Old French), to flirt, to behave lasciviously (13th cent. or earlier) < fol fool adj. In later use re-formed < folly n.¹Compare Italian †folleare (13th cent.).

Etymology: < fool n.¹ + -ly suffix². Compare foolishly adv.