This is a pictograph of a phallus. Hence, it is an ideograph for the adjective male or the noun man.
Illustrative Text References:
Harappa: Tablet: H-942 A: Sayid Ghulam Mustafa Shah and Asko Parpola, 1991: Corpus of Indus Seals and Inscriptions: Volume 2: Page 340: Collections in Pakistan: Suomalainen Tiedeakatemia.
Lothal: Seal: L-39 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 246: Collections in India: Suomalainen Tiedeakatemia.
Image Credit:
Indus Script Sign Number 211: Sign List of the Indus Script: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.