From Longman Dictionary of Contemporary Englishcollarlesscol‧lar‧less /ˈkɒlələs $ ˈkɑːlər-/ adjective a collarless jacket, shirt etc is one that does not have a collar
Examples from the Corpus
collarless• He dressed differently too - he wore a collar and tie while their shirts were collarless.• In baggy collarless cream shirt and hip-hugging chino trousers he looked so overwhelmingly attractive that she couldn't drag her eyes away.• He was wearing a waist-length possibly leather jacket, pale blue jeans, a light collarless shirt and white trainers.• She noticed his collarless shirt was faintly grimy and there was a button missing.• He had a grey walrus moustache and was wearing a collarless shirt with the sleeves rolled up.• His clothes - a Fair Isle pullover and collarless white shirt - were quaintly old-fashioned. 