The correct translation is 'maiden'. Slender and tall is incorrect. Mae'n hir translates to 'she is long' so someone is adding some humour to the meaning. [noted -ed]
Meinir was the tragic bride of Rhys Mawredog in the Welsh fable, Rhys a Meinir. In Nant Gwrtheyn, a North Welsh village, the two were set to get married. As is the local tradition there, the bride had to hide from her soon-to-be husband. She hid in a tree, and Rhys was supposed to find her. He couldn't, however, and Meinir died in the tree. One stormy night, the tree was cracked open by lightning. Out fell a skeleton - in its wedding dress! I would know, by the way - I go to a Welsh school, and we did Rhys a Meinir in Year Seven as our Chwedlau topic. I played Meinir in the play!