What is the meaning of Em-?

The form taken by en- before the labial consonants b and p, as it assimilates place of articulation.

em-, Alternative form of en-

em- (form of en- before b, m or p)

The first element of a few compound pronouns and adverbs. The compounds were formed by syntagmatic fusion, the role of the prefix is ​​emphasis.

Class 9 adjective concord; form of en- used before stems beginning with a labial consonant.

you, thou (second-person, singular, subject proclitic pronoun)

your, thy (second-person, singular, possessive pronoun)

forms verbs indicating motion or transformation into the prefixed noun; in-

Alternative form of en- used before b or p; em-

Class 9 adjective concord; form of en- used before stems beginning with a labial consonant.

Class 9 adjective concord; form of en- used before stems beginning with a labial consonant.

Source: wiktionary.org