Abbreviation of まつ毛エクステンション. Common in beauty salons and everyday conversation.
See also: まつげエクステンション
I plan to go get eyelash extensions next week.
When you get eyelash extensions, your eyes look bigger and brighter.
Clipped from まつ毛エクステンション, itself from まつ毛 (eyelash) + エクステンション (extension).