Busting is a
common adjective used to describe when someone experiences a very
strong urge to go to the
toilet.
The term is more likely to be associated with an urgent need to wee, however it can also be used to describe an urgent need to poo.
Busting is widely considered the highest possible urge level at which a person can remain without weeing or pooing their pants.
Young
children are likely to hold on to their wee for ages, so that by the
time they decide to go to the
toilet, they realise they are busting, and have to run in order to not wee their pants