The modern habit of shaving underarms only started during WW1.

For some ladies; another reason to hate wars. Prior to the First World War, women only shaved their scalp and (as I’m sure many denied) their facial hair. But around 1915, the sleeveless dress came into the picture, and women’s underarms became yet another part of their bodies that marketers could bully them about. 

According to Hope magazine, the anti-underarm hair marketing campaign began with an ad in Harper’s Bazaar (a magazine aimed at the upper crust). It featured a photograph of a young woman dressed in a slip with a toga-like outfit over one shoulder, and her arms up to reveal completely clear armpits. 

The ad read “Summer Dress and Modern Dancing combine to make necessary the removal of objectable hair.” (catchy isn’t it?). Within three months the once unmentionable word, “underarm” became a household term and the removal of its hair became a routine task. 

(Source)