formalwear consisting of full evening dress for men
"The guests arrived in their tails, looking dapper at the royal gala."
plural of tail
"The tailor carefully trimmed the tails to give the coat a sharper, more modern silhouette."
Clipping of tailcoat.
Third-person singular simple present indicative form of tail
"The detective's car slowly tails the suspect through the crowded market."
In plain English: To tail someone means to follow them closely, usually secretly so they don't notice you are watching.
"The dog tails its owner all over the house when he comes home from work."