Other forms: straitened; straitening; straitens
To straiten is to restrict financially, the way having your rent increase can straiten your circumstances if your salary stays the same.
The verb straiten is an old-fashioned one, but it's one way to describe a situation that's been diminished or constrained, usually because of money problems. This meaning derives from an earlier definition, "to restrict or make narrow," from strait, "narrow." If you find your circumstances narrowed by a lack of finances, you'll know what this word is all about.