[RWBY Volume 3&4 Spoilers]
Theory: Qrow's semblance is not actually "bad luck." It is an (admitted badly controlled) inhibiting aura that manifests as mild intoxication in others.
Justification: All semblances encountered up to this point manifest in a manner that can be (at least conceivably) fully and analytically quantified. Examples:
- Ruby's semblance is speed with a side effect of rose petals being generated. She likely has a maximum speed, velocity/time aura burn rate, number of petals/time petal generation rate, etc.
- Sun's semblance allows him to generate semi-autonomous energy clones. He likely has a maximum distance for control, (number of clones * distance)/time aura burn rate, clone hit points, etc.
- Weiss's semblance allows her to summon glyphs which exert specific kinematic (pushing/pulling)/temporal (time dilation) effects. She likely has a maximum number of glyphs that can be summoned at a time, number of glyphs/time aura burn rate, intensity of gylphs (e.g., how much force a pushing glyph applies) aura burn rate, etc.
Qrow's semblance (as described) however... Is bad luck? A person's luck isn't exactly quantifiable. Are we to believe that if Qrow went walking through the floor of a casino all current players would be doomed to lose all their money? This seems silly, and frankly doesn't match the scope of the rest of the RWBY-verse world building. And yet, how do people perceive luck in their day-to-day lives? If someone who is not normally klutzy slips on a patch of ice, falls, and breaks their arm, were they incompetent or unlucky? Most would probably say unlucky. However, subtle actions we take, or decisions we make, can play into how "unlucky" we get.
Theory expanded: Qrow's semblance causes individuals around him to manifest symptoms similar to those of being intoxicated. Examples include: impaired judgement, impaired reaction timing, impaired coordination.
Let's look at some examples in Volume 3 and 4:
Volume 3, Episode 2 - Qrow has been sitting in a bar watching the tournament matches on TV. As he is leaving, the bartender knocks over a glass and breaks it. A bartender with a proclivity for breaking glasses probably wouldn't be employed long, so we can presume this is an atypical moment for him. Bad luck? Or impaired coordination?
Volume 3, Episode 3 - Qrow needles Winter into a massive throw down outside of the school. This causes significant property damage and gets Winter in trouble with her superior. Winter is an officer in the Atlesian Military (and presumably an at least modestly ranking one to be working so closely with General Ironwood). Is she really that short tempered and easily antagonized into a fight? Is she really that completely out classed by Qrow? (He is casually dodging her wild swipes while barely breaking a sweat...). Is she really that surprised to have Glynda, Ozpin, and Ironwood all turn up and reprimand her? This entire scene is an overwhelmingly embarrassing and undignified moment for Winter... And I would argue atypical for her. A momentary lapse in judgement and bad luck that she and Qrow were caught? Or possibly impaired judgment that lead to her being drawn into the fight so easily... impaired coordination and reaction timing that allowed Qrow to fight her with such ease... and further impaired judgement at being surprised by Ozpin and company's arrival (after... you know... destroying the entire court yard).
Volume 3, Episode 4 - Qrow completely destroys both Yang and Ruby in their Ninja Warrior game. It seems unlikely that Qrow plays this game often, while Yang and Ruby probably regularly play each other. Why are both of them so easily outclassed? Bad luck, or impaired reaction time?
Volume 4, Episode 7 - Qrow and Tyrian fight and Ruby forces herself into the mix. She nearly gets clobbered by a falling 4x4, only to be rescued by Qrow (to his own near end). This is probably the only example we can point to of what might be some sort of mystical bad luck generated by Qrow... But bad luck also just happens. Did Qrow's semblance somehow cause a 4x4 to fall? Or is it more likely that Ruby's judgement was impaired leaving her more gung-ho than usual, and diving headlong into a fight that almost certainly was going to get her hurt (or killed... or kidnapped) regardless of falling 4x4s?
Glynda may be right that Qrow's always drunk, but maybe he's just trying to stay in good company with everyone around him...