From an unused root probably meaning, to be stout; the abdomen (as obese) -- fifth (rib).
Englishman's Concordance
1) belly, abdomen, fifth ribs Part of Speech: noun masculine A Related Word by BDB/Strong’s Number: from an unused root probably meaning, to be stout Same Word by TWOT Number: 687a
Brown-Driver-Briggs
II. חֹ֫מֶשׁnoun masculinebelly (Aramaic ) — הַחֹמֶשׁ2 Samuel 2:23 3t.; ׳וַיַּכֵּהוּ ֗֗֗ אלהֿח2 Samuel 2:23and he smote him in the belly; 2 Samuel 20:10 + 2 Samuel 4:6, read וַתָּ֫נָםוַתִּישָׁ֑ןand she slumbered and sleptᵐ5 We Dr.; yet corruption difficult to explain, see Klo; ׳וַיַּכֵּהוּ שָׁם הח2 Samuel 3:27 (where probably insert אלֿ compare Dr).
IV. חמשׁ (√ of following; meaning dubious; perhaps compare Arabic army, Sabean חֿמס men of a tribe who can bear arms; √ then possibly=I. חמשׁ (e.g. army as composed of five parts) see Lane Frey; also SabDenkm24, which cities tribus (from tres), quartier (from quartus, quatuor); > Thes MV and others who compare Arabic be firm strong, be courageous, etc.)
2 Samuel 2:23 HEB:הַחֲנִ֜ית אֶל־ הַחֹ֗מֶשׁ וַתֵּצֵ֤א הַֽחֲנִית֙ NAS: struck him in the belly with the butt KJV: him under the fifth [rib], that the spear INT: of the spear under the belly came the spear
2 Samuel 3:27 HEB:וַיַּכֵּ֤הוּ שָׁם֙ הַחֹ֔מֶשׁ וַיָּ֕מָת בְּדַ֖ם NAS: he struck him in the belly so that he died KJV: and smote him there under the fifth [rib], that he died, INT: struck and there the belly died of the blood
2 Samuel 4:6 HEB:וַיַּכֻּ֖הוּ אֶל־ הַחֹ֑מֶשׁ וְרֵכָ֛ב וּבַעֲנָ֥ה NAS: and they struck him in the belly; and Rechab KJV: and they smote him under the fifth [rib]: and Rechab INT: struck in the belly and Rechab and Baanah
2 Samuel 20:10 HEB:בָ֨הּ אֶל־ הַחֹ֜מֶשׁ וַיִּשְׁפֹּ֨ךְ מֵעָ֥יו NAS: so he struck him in the belly with it and poured KJV: so he smote him therewith in the fifth [rib], and shed out INT: struck in the belly and poured his inward
) — הַחֹמֶשׁ2 Samuel 2:23 3t.; ׳וַיַּכֵּהוּ ֗֗֗ אלהֿח2 Samuel 2:23and he smote him in the belly; 2 Samuel 20:10 + 2 Samuel 4:6, read וַתָּ֫נָםוַתִּישָׁ֑ןand she slumbered and sleptᵐ5 We Dr.; yet corruption difficult to explain, see Klo; ׳וַיַּכֵּהוּ שָׁם הח2 Samuel 3:27 (where probably insert אלֿ compare Dr). 
army, Sabean 
be firm strong, 
be courageous, etc.)