Sun phat 

A mild (semi) to wild (full girth) errection as a result of solar energy from direct sun light beaming directly onto your groin.
Occurs unexpectedly, often on public transport.

Slacks turn to tights in the sun light.

Sun phat on the bus this arvo was glorious but also inconvienient and awkward.

Sun phat by Esskaybee July 21, 2021