Last but not Least: more important, but not in order of mention or occurrence
Example: I would like to thank my teachers and friends, and, last but not least, my family.
Last (final) straw: The final problem in a series of problems.
Example: Losing my job was bad enough, but being evicted was the final straw.
Lend Me Your Ear: to politely ask for someone to listen carefully
Example: Friends, Romans, countrymen, lend me your ears. (Julius Caesar by William Shakespeare)
Let Sleeping Dogs lie: said to warn someone that do not disturb a situation as it is
Example: We would be better to let sleeping dogs lie and not discuss any more.
Let the Cat out of the Bag: to share a secret information that shouldn't be known by others
Example: It should be a serious confidential information until someone let the cat out of the bag.
Liquor Someone up: to get someone drunk.
Example: My friend used to have liquor his client up on whiskey when they first met
long story short: come to the point - leave out details
Example: Long story short, Mike did indeed call for government financing, but only after bankrupcy.
looked down in the dumps: looked depressed or sad
Example: She certainly looked down in the dumps.
loose cannon: An unpredictable or uncontrolled person who can cause damage if not kept in check
Example: As it turned out, John is not just a loose cannon, but he makes sense.