“So mote it be” is a ritual phrase used by the Freemasons, in Rosicrucianism, and more recently by Neopagans. Some of the younger Masons have started to abbreviate it to “SMIB”.

The word “mote” is an archaic Old-English verb and can be translated as “may”. Therefore, “so mote it be” means “so may it be”. The direct translation of the word “Amen” is “so be it”. 

This phrase appears first in the Halliwell or Regius Manuscript, the earliest known document relating to a society of Masons in England, dating from the first half of the 15th century. “Amen! amen! so mot hyt be! Say we so all per charyté”.

Freemasons end their prayers the same way they did in 1390. The next time that you are in Lodge and say, “So mote it be”, you are reminded that you are continuing a 600 year-old Masonic tradition.

More Info:
https://scottishrite.org/scottish-rite-myths-and-facts/qa-so-mote-it-be/
https://en.wikipedia.org/wiki/So_mote_it_be