"In the original Parc Place image, the glyph of the underscore character (_) appeared as a left-facing arrow (like in the 1963 version of the ASCII code). Smalltalk originally accepted this left-arrow as the only assignment operator. Some modern code still contains what appear to be underscores acting as assignments, hearkening back to this original usage. Most modern Smalltalk implementations accept either the underscore or the colon-equals syntax."

https://en.wikipedia.org/wiki/Smalltalk#Assignment