Grow Legs 

Something that's likely to be stolen.

Keep an eye on your suitcase, it's liable to grow legs.

Grow Legs by Bladi February 26, 2008

Don't let it grow legs 

Means to not let a situation, feeling or idea get out of control...

X= "I've been having this feeling for my co-worker and I know it's wrong 'cause I'm married"

Y= "Wow man, I think it's serious, don't let it grow legs"

Don't let it grow legs by Darkestapathy February 7, 2007

grow arms and legs 

To become substantially more complicated, especially when this was unforeseen. Colloquial expression, primarily used in (British) English. Origin: by analogy to human biological development in the womb.

The project seemed simple at first, but now it's starting to grow arms and legs.

grow arms and legs by mattgemmell September 1, 2015