This symbol appears to be a combination of the verb to join and a pictograph of a walking stick. It may be related to the Chinese character pū 攴, which means to rap or tap. In Indus script, it is an ideograph for the verb to rap.
Illustrative Text Reference:
Mohenjo-daro: Seal: M-331 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 84: Collections in India: Suomalainen Tiedeakatemia.
Comments:
Mahadevan may have assumed that M-331 was a tablet rather than a seal. The orientation of sign number 1471 therefore differs from that in Joshi and Parpola.
Image Credit:
Indus Script Sign Number 1471 (mirrored): List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.