This is a modified version of the verb to join. One half of the symbol has been replaced by an arrow pointing to the left. The implication is that the arrow is travelling away from the join.
Hence, this symbol is an ideograph for the verb to take one's leave or to depart.
Illustrative Text Reference:
Harappa: Seal: H-7 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 163: Collections in India: Suomalainen Tiedeakatemia.
Indus Script Sign Number 4008 (To Depart): List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.