Definition: to do something that others may disapprove of. This disapproval is often serious, but can also be mock or humorous disapproval. [British]
E.g.1. I don’t want my boyfriend to go on holiday on his own. I am worried about what he might get up to.
E.g.2. The children are very quiet upstairs; I wonder what they’re getting up to.
E.g.3 Hi Paul! How are you? What have you been getting up to recently?
This phrasal verb can’t be separated.
Source: Knight's English