Squash up
Definition 1 of 2
Pro Tip 1/3Recycling Context
Often used when talking about compressing waste like paper or plastic for recycling. 
Squash up the plastic bottles before you throw them away.
Pro Tip 2/3Space Saving
Implies making more space by compressing items. 
If we squash up the clothes, we can fit more in the suitcase.
Pro Tip 3/3Intentional Damage
Suggests the act is deliberate, not accidental. 
She told her brother to squash up the box so it wouldn't be usable again.