This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
牫 combines 牜 (cow/ox) and 戈 (halberd/weapon). The exact historical development is uncertain, but it likely originally related to cattle and weaponry or slaughter.
A cow (牜) beside a halberd (戈) suggests something involving cattle and a weapon, but the exact meaning is unclear.
For ハツ, imagine a cow (牜) being struck by a halberd (戈) with a sharp 'hats!' sound: hats -> ハツ.