The word "whilstsynonym" appears to be a typographical error. It seems likely that you meant the word "whilst." "Whilst" is a conjunction that means "while" or "during the time that." It's often used in British English and can imply simultaneous actions or conditions.
"Whilst" originates from the combination of "while" and "st," which is a suffix used in Old English. The term has been used since the 14th century and is chiefly found in British English.