This is the 182nd most frequent Arabic word.
ليست
"ليست" means "is not" (feminine).
The word 'ليست' is used here in its negation function for the nominal sentence. 'ليست' is the feminine form of 'ليس', used because 'السيارة' is feminine singular.
هذه السيارة ليست غالية.
This car is not expensive.
Here, 'ليست' is employed to negate an adjective describing the subject in a nominal sentence. 'سماء' (sky) is treated as feminine in Arabic, so the feminine 'ليست' is used.
السماء ليست صافية اليوم.
The sky is not clear today.
In this context, 'لستُ' (a derivation with the first-person singular suffix) is used referring to the speaker's doubt, showcasing the verb's conjugation matching the subject's gender and number.