'assegai' (base of 'assegaied') originates from Portuguese, specifically the word 'azagaia', which denoted a spear or javelin.

'azagaia' (Portuguese) entered European languages through contact with African and Islamic cultures; it was borrowed into English as 'assegai' (recorded from the 16th–17th century) and the verb forms (e.g. 'assegaied') derive from that noun.

Initially, it meant 'a light spear or javelin'; over time the noun retained this meaning, and English also developed verbal uses meaning 'to wound or kill with an assegai', which is the sense behind the past form 'assegaied'.