At least

smallest in size, amount, degree, etc.; slightest:
He gave the least amount of money of anyone.
lowest in consideration, position, or importance.
something that is least; the least amount, quantity, degree, etc.
South Midland U.S. the youngest in a family or group.
to the smallest extent, amount, or degree:
That’s the least important question of all. He talks least.
at least,

at the lowest estimate or figure:
The repairs will cost at least $100.
at any rate; in any case:
You didn’t get a good grade, but at least you passed the course.

Also, at the least.
not in the least, not in the smallest degree; not at all:
I am not in the least concerned about the outcome of the World Series.
determiner

the least, the superlative of little you have the least talent of anyone
(as pronoun; functioning as sing): least isn’t necessarily worst

at least

if nothing else: you should at least try
at the least

at the least, at least, at the minimum: at the least you should earn a hundred pounds
(usually used with a negative) in the least, in the slightest degree; at all: I don’t mind in the least
adverb
the least, superlative of little they travel the least of all
adjective
of very little importance or rank
adj.

Old English læst, earlier læsest “smallest” (superlative of lytel “small”), from Proto-Germanic superlative *laisistaz (see less). Qualifying phrase at least is Middle English æt læstan. As a noun, from early 12c.; as an adverb, c.1200.
Also, at the least. According to the lowest possible assessment, no less than. For example, At least a dozen more chairs are needed, or The job will take four hours at the least. [ c. 1050 ]
Anyway, anyhow, as in At least you got there on time, or The children enjoyed the dessert at least. [ c. 1050 ]
For synonyms, see in any case

least of all
least resistance, line of

also see:

at least
in the least
last but not least
to say the least