What sis the word sinsister mean before evil and wicked?

The Word 'Sinister' comes from medaieval French, and means 'LEFT'.

So if you are a left handed person, you are a SINISTER person.

Conversely, if you are a right handed person, then you are a DEXTER person. (as in 'dexterous').

In heraldic crests , anything that is left=handed is described as Sinister.

The modern French Word for 'left' is 'gauche' , used for someone who socially lacks confidence/awkward.