From the root ب ط ن (b ṭ n). Cognate with Hebrew בֶּטֶן (béten, “stomach”), Aramaic בטנא (baṭnā, “womb”), Ugaritic 𐎁𐎉𐎐 (bṭn, “to be pregnant”).
Practice speaking and memorizing "بطن" and many other words and sentences in Arabic.