From the second edition (1989):
(‖ˈhaːtʃɛk, ˈhæ-) Also hacek. [a. Czech, dim. form of hák hook.]

A name for the diacritic ˇ, which is used in Baltic and Slavonic languages.
When placed above a consonant, it indicates palatalization (as č (tʃ), š (ʃ)), etc.); when placed above e, it indicates the vowel phoneme jatʹ.

1953 W. E. Harkins Mod. Czech Gram. 1 The Latin alphabet has been expanded‥through the use of diacritical marks, called the čárka ʹ, kroužek ˇ, and háček °.‥ The single vowel with the háček, ě, is sometimes listed with e. 1959 W. R. & Z. Lee Teach Yourself Czech p. xv, All consonant letters marked with a hook (háček)‥are ‘soft’. 1980 R. Auty in Schenker & Stankiewicz Slavic Literary Lang. 169 Hus's lozenge-shaped dot was changed to the ‘hook’ (háček) which lives on as the reversed circumflex of the present-day Czech alphabet. 1984 E. Stankiewicz Grammars & Dict. Slavic Lang. 3 Hus replaced the medieval system of digraphs with one of diacritics, among which the dot (later replaced by a háček) marked the palatals.