What is a suffix for locked up?

There isn't a single, perfect suffix for "locked up" as a standalone word. However, depending on the intended meaning and context, you could consider these options:

For a general sense of confinement or imprisonment:

* -ed: Locked-up (e.g., "He was locked-up for years.")

* -ing: Locking-up (e.g., "The police were locking-up the criminals.")

For a more specific meaning:

* -ment: Lock-up (e.g., "The prisoner was held in a lock-up.")

* -ion: Lock-up (e.g., "The lock-up of the criminals was swift.")

* -age: Lock-up (e.g., "The police used the lock-up for processing suspects.")

Other options:

* -ed up: Locked-up (e.g., "The suspect was locked-up tight.")

* -in: Lock-in (e.g., "The employees had a lock-in at the office.")

The best suffix will depend on the specific context and desired meaning.