I read a sentence in a book, The Word Power Made Easy, which was
You are boastful to the point of being obnoxious - you have only one string to your conversational bow, namely, yourself; and on it you play a number of monotonous variations.
According to most of the sources online relating to the the idiom, the word bow means the archer's bow in this idiom but then why has the author of the sentence used "playing a number of monotonous variations" after the idiom (you don't play something through a bow). I am wondering if there is any sense of the word "bow" that relates to an instrument because it is through an instrument's string that something is played.