1. very big, enormous
'Massive' is an adjective used to describe something huge in terms of size, amount or mass. "Are you going to eat all that? The portion is massive." "We have had to commit to a massive reduction in spending." It is a common adjective used in a variety of contexts.
2. brilliant, excellent, lovely
sth is only massive
In Irish and UK colloquial slang 'massive' can be used as an adjective to say that something is great or brilliant. This has a very positive connotation but is very informal, so this use of 'massive' is usually used in social contexts among younger people.