In general, there are two meanings to this term.
1) A swearer is someone who has taken an oath or made a promise of a particular kind. It doesn't matter what type of oath the person has take or the circumstances which it was taken under they are still referred to as 'a swearer'
2) A swearer in this case is a person who uses colorful language to express their inner feelings when they feel polite or less vulgar language can't completely express their emotions, feelings and thoughts to the same intensity as the former.
1) Daniel just got his American citizenship last weekend. Congratulations to him, he is how an american swearer.
2) I promise to be a good person to you... i really want to be your swearer
3) Daniel was mega annoyed because the guy at the store bumped in to him and Daniel was enraged, immediately becoming a swearer