What is the origin of 'be my guest' idiom?
The OED defines it as you are welcome to (something); do as you wish. The earliest citation is dated 1955. It doesn’t seem weird to me. The speaker is inviting the other person to feel comfortable about what is being asked. I imagine it would occur predominantly in speech, but it would not necessarily be out of place in informal writing either.
I am not master so that I can explain you, but this link I am giving below may help you
http://www.phrases.org.uk/bulletin_board/57/messages/369.html
good luck.