lest

Lest has a similar meaning to in case (see here) or so that … not (see here). It is rare in British English, and is found mostly in older literature and in ceremonial language. It is a little more common in formal American English.

• They kept watch all night lest robbers should come.
• We must take care lest evil thoughts enter our hearts.

Lest can be followed by a subjunctive verb (see here).

• The government must act, lest the problem of child poverty grow worse.