Least is the superlative degree of little; less is the comparative: "Toby has less money that I have; in fact, she has the least money of any girl in our group." Both least and less always refer to amount, size, or importance: "less value," "least importance," "least influence," "less distance." (For the distinctions between less and fewer, See fewer.) "At least" and "not in the least" are frequently mere fillers in conversation and are overworked phrases. Neither least nor less should be confused with the conjunction lest, which means "for fear that": "He kept moving his fingers lest they freeze in the bitter cold." Lest also conveys the meaning of "so that" when used to introduce a statement suggesting a need for caution: "Don't say much lest the opposition steal your plan."
Dictionary of problem words and expressions. Harry Shaw. 1975.