From arse + -le.
arsle (third-person singular simple present arsles, present participle arsling, simple past and past participle arsled)
Probably a back-formation from arslet., definite form of arsel, with the same meaning, from an Old Swedish contraction of ars (cognate with English arse) + hol, hul (cognate with English hole), identical to English arsehole.
arsle n