This is the 4752nd most frequent German word.
zulegen
"Zulegen" means "to acquire, gain, add, or purchase."
Here, 'zulegen' is used to signify gaining or increasing something, in this case, weight.
Er hat im letzten Jahr ordentlich an Gewicht zugelegt.
He has gained quite a bit of weight last year.
In this sentence, 'zulegen' refers to the action of acquiring or purchasing something, specifically a car.
Sie will sich ein neues Auto zulegen.
She wants to acquire a new car.
In this context, 'zulegen' implies adopting or developing an attribute or characteristic, metaphorically represented as 'a thick skin'.