LHM
LHM has 2 meanings
Lord have mercy
LHM is an acronym that stands for "Lord have mercy." It is an expression typically used when in a state of disbelief from something good or bad.
The expression comes from the feeling that you can't take anymore of something, whether it be good or bad, and you're asking for it to stop. Some instances where you may see the acronym is in response to a text containing outrageous news or a post online with a shocking picture. The acronym may also be used literally by a religious person that is asking for mercy from God.
Example
She just had another baby, which makes an even eight children now
LHM
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Desperate and/or exasperated people use LHM to stand for "Lord, help me." This acronym is a plea for whatever god the person believes in to give them strength.
People often use LHM when describing a difficult situation or their response to it. For example, when talking about a particularly nasty co-worker, your friend may say "LHM, I wanted to slap him and quit right then and there."
Typically, LHM refers to the Christian or Hebrew God. Some people, however, may use LHM to refer to a different deity or their own sense of inner strength.