What is another word for not strong?

The best word to use depends on the context. Here are some options:

For physical strength:

* Weak

* Frail

* Delicate

* Feeble

* Unstable

* Flimsy

* Paltry

* Slight

* Brittle

For mental or emotional strength:

* Vulnerable

* Fragile

* Unstable

* Unsure

* Hesitant

* Timid

* Indecisive

* Unassertive

For general strength:

* Insufficient

* Inadequate

* Limited

* Poor

* Weak

* Flawed

* Deficient

* Inferior

More creative options:

* Ephemeral (short-lived)

* Tenuous (weak and uncertain)

* Evanescent (fading or disappearing)

Please provide more context if you'd like a more specific suggestion!