Searching at Google Books I found this blurb, quoted from Stars Who Made the Halls, by Sidney Theodore Felstead:

Where did you get that boko, Uncle,
Is it your nose, or a big carbuncle,
Hide it under your handkerchief, or cover it with your hat,
The driver will take it for a danger signal,
Don't stick it out like that.

Unlike what you may have been thinking (for shame!), boko was (is?) apparently a slang word for nose, a propos of which:

"Barney Boko was a fictional character in an early comic strip in The Dandy. It was about a tramp whose incredibly long nose could be used for anything, from a christmas tree to a bridge."