Core sense — he made a thing necessary; it was soft and crumbly; he ate crumbsgenerated
5.1
He made a thing necessary, or unavoidable.
5.1.1he wished such a one good, or he augured good for him—تحتّم لِفُلَانٍ بِخَيْرٍ
Perhaps from the meaning above.
5.2it is soft, yielding, crummy, or easily broken—هُوَ ذُو تَحَتُّمٍ
Said of a thing eaten: it was soft, yielding, crummy, or easily broken.
In the S, التَّحَتُّمُ is said to mean الهَشَاشَةُ; but a marginal note states that something has been left out of this explanation, the right rendering being هشاشة الشَّىْءِ المأْكُولِ.
Explained in the K by هَشَاشٌ, which is evidently a mistranscription for هَشَاشٌ, synonymous with هَشٌّ.
it is fresh, juicy, or moist in its soft, crumbly nature—هُوَ غَضُّ المُتَحَتَّمِ
مُتَحَتَّمٌ is a regular maṣdar.
5.2.1
Said of a ثُؤْلُول [or wart] that had become dry: it crumbled, or broke into small bits.
Said of a glass vessel: it broke in pieces, one part upon another.
5.2.2
He ate a thing that was soft, yielding, crummy, or easily broken, in his mouth.
5.2.3he ate the food left on the table, or what fell from it during eating, of crumbs of bread and the like—حُتَامَة
A tradition says that one who eats and does this will enter Paradise.
5.3he became cheerful, brisk, lively, or sprightly because of such a thing—تحتّم لِكَذَا
Show Lane's original text
He made a thing to be necessary, or unavoidable. (K, * TA.) [Hence, perhaps,] تحتّم لِفُلَانٍ بِخَيْرٍ He wished such a one good: or he augured good for him. (K.) See also 7. [It (a thing that was eaten) was soft, yielding, crummy, or easily broken.] It is said in the S that التَّحَتُّمُ signifies الهَشَاشَةُ : but in a marginal note it is stated that there is an omission in this explanation, the right rendering being هشاشة الشَّىْءِ المأْكُولِ . (TA.) One says, هُوَ ذُو تَحَتُّمٍ [It is soft, yielding, crummy, or easily broken: explained in the K by هَشَاشٌ , which is evidently a mistranscription for هَشَاشٌ , syn. with هَشٌّ ]. (S, K.) And هُوَ غَضُّ المُتَحَتَّمِ [It is fresh, juicy, sappy, or moist, in its soft, yielding, crummy, or crumbling, nature; مُتَحَتَّمٌ being a regular inf. n.] (S, K.) It (a ثُؤْلُول [or wart] when it had become dry) crumbled, or broke into small bits. (TA.) And It (a glass vessel) broke in pieces, one part upon another. (TA.) He ate a thing that was soft, yielding, crummy, or easily broken, in his mouth. (Lth, K.) He ate the حُتَامَة , i. e., the food that remained upon the table, or what fell from it during eating, (K, TA,) of the crumbs of bread &c. (TA.) It is said in a trad. that he who eats and does this will enter Paradise. (TA.) تحتّم لِكَذَا He was, or became, cheerful, brisk, lively, or sprightly, by reason of such a thing. (K.)