*1.to not allow your feelings to show; to not express how you really feel
If you hold in an emotion or feeling, you do not allow yourself to express it, often making it more difficult to deal with.
These children hold in their feelings so as not to upset their parents.
Go ahead and cry. Don't hold it in.
As for the watchers, the Easterner’s held-in breath burst from him in relief, pure relief after the anxious waiting.
learningen
*Synonyms
(To stop, control or not show emotions)
manage, suppress. deal with ...
*Antonym
let something out
*2.to stop something from moving outwards; keep from being perceived by others
I was trying desperately to hold my tummy(=stomach) in.
*Synonyms
(To stop someone or something from moving)
hold back, intercept, obstruct ...
ref: macmillandictionary.com, vocabulary.com, collinsdictionary.com, oxfordlearnersdictionaries.com