Longish means fairly long — not short, but not strikingly long either. A longish meeting, a longish walk, longish hair. The little tail -ish here means “somewhat,” the same softening you hear in youngish or tallish. It is casual, spoken English more than careful writing; a news report would more often say rather long or quite long. Though actually, -ish does another job in words like childish (“like a child”). In longish it does not mean “like a long.” It only turns the volume down on long.