"Colli pwysau"

Translation: To lose weight

[shwmae]

Remember pwys doesn't mean weight:

pwys = a pound (Imperial unit of weight)

pwysau = pounds (plural of above); weight (in general)

[DaibhidhR]

Interesting. It did - see GPC, but clearly doesn't now. I wonder when and why it changed to pwysau

[ThomasoFflandrys]

To gain weight = ennill pwysau?

[ibisc]

Yes

[shwmae]

You can use magu pwysau lit. “to nurture weight” to mean the same thing too.