S výrazem “I'm trousers in …” jsem se já, dříve narozený
Angličan, nikdy nesetkal ale dá se to pochopit tak, jak jste popsala. Bude to
varianta podobného idiomu to be pants at sth:
pants adjective [not before noun] British English, spoken,
informal: very bad
“He’s pants”. The first time I heard this I thought
it was hilarious. It is used to indicate that something is worthless garbage (or
“rubbish”, as the Brits say). It can refer to a person, an action, a thing,
or a place, so it can be “your argument is pants” or “my workplace is
pants” or “I’m pants at maths”. (dailykos)
Google can't find any example (in this meaning) of “I'm trousers in”, so
you can safely forget this expression. You'll never need it.