The name Mahlah in the Bible
There are two Mahlahs in the Bible; one is a woman but the other one we're not sure about. Actually, the only reason we're not sure is that it's not explicitly said that the mystery Mahlah is a lady, and in Scriptures, that's usually suspicious. But, all suspicion aside, the name Mahlah is the feminine version of the masculine name Mahli, so that gives us a clue that this might be a feminine name. On the other hand, there are two words in Hebrew that look just like this name, and one is masculine and the other is feminine. The bottom line is that we don't know whether Mahlah the child of Hammoleketh (who is probably a granddaughter of Manasseh) is a boy or a girl (1 Chronicles 7:18).
Etymology of the name Mahlah
The name Mahlah comes from the root group חלה (hala I, II & III):
The name Mahlah may come from any of these verbs and nouns but for some reason the consulted sources all prefer the root that means to be sick. It may seem odd that in antiquity people would call their children after sickness but it should be noted that in the times of the Bible names worked differently than in our day and age. By naming his daughter Sickness, Zelophehad probably didn't want to say that he thought his daughter was a disease, but perhaps that she was born during a period of infirmity, or else to remind himself and others that sickness is always among us. Compare for instance the meanings of the names of the children of Hosea: Jezreel (God sows — 1:4) and Lo-ruhammah (no mercy — 1:6), or Isaiah's son Maher-shalal-hash-baz (swift is the booty, speedy is the prey — Isaiah 8:3).
For a meaning of the name Mahlah, both NOBSE Study Bible Name List and Jones' Dictionary of Old Testament Proper Names read Disease.