The headword, which has the same glossing in other lexica, is accusative case. It is quoted (as Latte notes on Hesychius s.v.) from Homer, Iliad 22.146; cf. the scholia there.
[1] Literally, one on which it is possible to travel in a wagon or cart, ἅμαχα (alpha 1486).
[2] A sophist and historian of the fourth century CE: see mu 120. The quotation is not from him, however, but from a much earlier writer (see next note), and indeed the word ἁμαξιτός is common enough in all periods.
[3] Xenophon, Anabasis 1.2.21 (see link below).