Casual and affectionate - used when talking about babies, kids, or in playful contexts about one's own bare feet. The fuller form 脚丫子 (jiǎoyāzi) is even more colloquial. For neutral or formal contexts, use 脚 (jiǎo).
memory
丫 originally pictures a forked tree branch ('Y'-shape) and is used metaphorically for things that fork - like the gap between toes. So 脚丫 evokes 'the forked-toe foot'.
