Generally used to mean scared of. Official meaning is an extreme or irrational fear of or aversion to something.
1786, "fear, horror, aversion," Mod.L., abstracted from compounds in -phobia, from Gk. -phobia (in widespread popular use with native words from c.1800), from phobos "fear," originally "flight" (still the only sense in Homer), but it became the common word for "fear" via the notion of "panic, fright" (cf. phobein "put to flight, frighten"), from PIE base *bhegw- "to run" (cf. Lith. begu "to flee," O.C.S. begu "flight," bezati "to flee, run," O.N. bekkr "a stream"). Psychological sense attested by 1895.