least 
(lēst)
(lēst)adj. A superlative of little.
1. Lowest in importance or rank.
2.
a. Smallest in magnitude or degree.
b. Slightest or tiniest: didn't care the least bit.
adv. Superlative of little.
To or in the lowest or smallest degree.
n.
Idioms: One that is the lowest or smallest in importance, rank, magnitude, or degree: The dinner menu is the least of my worries tonight. The least you can do is to be polite.
at least
1. According to the lowest possible assessment; not less than: We waited at least an hour.
2. Regardless of what has occurred or will occur; in any case: You might at least answer the letter.
3. Used to emphasize a somewhat positive aspect following a negative one: The results weren't encouraging, but at least they were accurate.
in the least
At all: I don't mind in the least.
least of all
Particularly not: Nobody cared, least of all the manager.
[Middle English, from Old English lǣst; see leis-2 in the Appendix of Indo-European roots.]