It means, in experience, that you have ideas above your station (as a woman).
Women are supposed to be meek. Meek women don't go into pubs alone.
Women are supposed to he subservient. Subservient women wouldn't be so bold as to walk into a pub alone.
Women are supposed to wait for a man to take control/recognise their authority. Women who goes into pubs alone don't recognise the authority of men.
Pubs are still regarded by many as a man's domain. Many people are suspicious of the morals of a woman who would go into a pub on her own.
Women are supposed to be mindful of their reputation. Women who are mindful of their reputation don't go into pubs on their own.
The stereotype is that pubs used to only be frequented by women with loose morals. Nice women wouldn't dream of going into a pub alone because of what it would say about her morals.
I've read posts on here in the past 12 months where a woman has been uncertain whether its OK for her to go into a pub on her own for lunch and a glass of wine of she's been visiting a other city for example. There are a lot of people (men and women) who still feel it inappropriate for a woman to go into a pub alone.
It can make some men feel insecure. If she is willing to go into a pub on her own, what other 'masculine' traits might she have? Might she be opinionated? Assertive? Not submit easily? Might she challenge him on poor behaviour? Might she disagree with him? Not very 'feminine' traits a lot of men still expect.
If a woman does something that steps outside the realm of 'women', they have 'othered' themselves in the eyes of some men and, at that point, they are unpredictable because it means they don't follow the rules.
Women, know your place.