Mei-mei used in the context with a secondary high tone (mae-mae), implies the following:
“hey there, hot stuff”
“hey there, sweet stuff”
It’s usually used in context of bars and night clubs in Asia where the girls are indeed working you for their pay.
Mei-mei used to describe a little sister is used with a sharp descending fourth tone. So even though the two words are written the same, your pronunciation could cause you to get slapped or a confused return expression.
Mandarin Chinese consists of four tones: first flat, secondary high, third deep, and fourth sharp. Two of the same words with a third tone makes the second word pronounced with a second intonation.
Little sister context (fourth tone):
Translation: "this is my mei mei"
Night club context (third and second tone):
Translaton: "hey mae mae, come cover here"
Lao (third tone) and Er (fourth tone) in Mandarin Chinese has the literal definition of "old two", but used in the context of the Chinese language means either:
(1) A person's male manhood (slang)
(2) The second oldest in the family
Slang context, usually used amongst males: "My Lao Er is sore after that hottie last night"
Normal context, when a parent is introducing their son or daughter: "This is the Lao Er of my family"