In standard English the primary meaning of the adjective bold is ‘brave, courageous, unafraid, daring’. This can shade into a related, negative sense of impudence, brazenness, or presumption. Another common sense is ‘visibly prominent, distinct, strong, or clear’, often associated with lines or colour. For nuance, compare the definitions by M-W, AHD, Oxford, Macmillan, Cambridge, and Collins.
When I first learned the word, though, it was in none of these senses: it meant ‘naughty, mischievous’. If I heard someone (including myself) described as bold, it meant they were misbehaving – or maybe being playful in a cheeky way. This is a very common usage in Irish English but absent from standard English; there’s no mention of it in the OED.
The sense is so intrinsic to the word in Ireland that when I read this line in Swing Time by Zadie Smith last week, I had to read it twice to be sure of the intent:
I’d been bold and walked into the church at the end of one of her rehearsals.
Smith means the word in the main standard sense. But I was primed to read the word Irishly because the narrator is a child, and bold in Irish English usually refers to misbehaving children (or puppies); the girl may even have been going to confession. Bernard Share, in Slanguage, quotes Lar Redmond in Emerald Square:
My poor mother had to spread her ration of love out too thinly and sometimes when I was bold – and that was often – there was none left over for me at all.
Bold may also be used by adults referring to themselves if they want to downplay the seriousness of their actions:
Or if they feel they’re being treated like children:
Also in the Irish Examiner, a few years ago, Ciara Flaherty described being bold as ‘a uniquely Irish way of misbehaving – otherwise known as “acting the maggot”, “kaffling” or “being a scut”.’ She shares this clip of Irish children talking about being bold:
The origins of the usage are not certain, but a couple of sources (Share’s Slanguage and Dolan’s Dictionary of Hiberno-English) suggest the influence of Irish dána, which straddles the senses: it can mean ‘daring, brazen,’ or ‘naughty‘ in reference to children.