This is the verb to join, which has been modified with an arrow pointing to the left. The left is associated with the west, the direction in which the sun gets weaker and eventually sets. 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.
Image Credit:
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.