The meaning of “בְּכִירָה”

“And it came to pass on the morrow, that the firstborn said unto the younger, Behold, I lay yesternight with my father: let us make him drink wine this night also; and go thou in, and lie with him, that we may preserve seed of our father.”

“Now the sons of Saul were Jonathan, and Ishui, and Melchi–shua: and the names of his two daughters were these; the name of the firstborn Merab, and the name of the younger Michal:”