The most notable use of this root was in the derivation of the name Q. Sangahyando “Throng-cleaver”, the name of a sword in Tolkien’s earlier writings back in the 1910s (QL/81) but ultimately becoming the name of a warrior of Umbar in The Lord of the Rings appendices (LotR/1048). The first iteration of this root was as ᴱ√HYAÐA “plough through” in the Qenya Lexicon of the 1910s, with derivatives like ᴱQ. hyanda “a blade, share” and ᴱQ. hyar “a plough” (QL/41). In the contemporaneous Gnomish Lexicon it had derivatives like G. hanna- “mow, cleave” (GL/48). In The Etymologies of the 1930s, it appeared as ᴹ√SYAD “shear through, cleave (through)” with derivatives like ᴹQ. hyar- “cleave”, ᴹQ. hyatse “cleft, gash” and N. hast “axe-stroke” (Ety/SYAD); it replaced a deleted variant ᴹ√KHYAD “cleave” (EtyAC/KHYAD).
However, in the first version of Tengwesta Qenderinwa (TQ1), also from the 1930s, Tolkien gave ᴹ√SYAD the gloss “compress” (PE18/51), and in the contemporaneous Outline of Phonetic Development (OP1) {ᴹ√KHYAD >>} ᴹ√SYAD had the derivative ᴹQ. hyarna “compact, compressed” (PE19/45). Similar forms appeared in the second version of Tengwesta Qenderinwa (TQ2) and Outline of Phonology (OP2) from the early 1950s (PE18/101; PE19/92). However, in between these two sets of documents, in the Quenya Verbal System of the 1940s, Tolkien gave ᴹ√SYAD the gloss “cleave through” as the basis for the verb ᴹQ. hyar- “cleave” (PE22/102).
The other element Sanga- of the name Sangahyando was derived from the roots √THAG or ᴹ√STAG, which Tolkien only ever gave glosses like “(com)press”. Thus I think it is best to assume the glosses in TQ1, OP1, TQ2 and OP2 were aberrations, and that √SYAD generally meant “cleave”.
The most notable use of this root was in the derivation of the name Q. Sangahyando “Throng-cleaver”, the name of a sword in Tolkien’s earlier writings back in the 1910s (QL/81) but ultimately becoming the name of a warrior of Umbar in The Lord of the Rings appendices (LotR/1048). The first iteration of this root was as ᴱ√HYAÐA “plough through” in the Qenya Lexicon of the 1910s, with derivatives like ᴱQ. hyanda “a blade, share” and ᴱQ. hyar “a plough” (QL/41). In the contemporaneous Gnomish Lexicon it had derivatives like G. hanna- “mow, cleave” (GL/48). In The Etymologies of the 1930s, it appeared as ᴹ√SYAD “shear through, cleave (through)” with derivatives like ᴹQ. hyar- “cleave”, ᴹQ. hyatse “cleft, gash” and N. hast “axe-stroke” (Ety/SYAD); it replaced a deleted variant ᴹ√KHYAD “cleave” (EtyAC/KHYAD).
However, in the first version of Tengwesta Qenderinwa (TQ1), also from the 1930s, Tolkien gave ᴹ√SYAD the gloss “compress” (PE18/51), and in the contemporaneous Outline of Phonetic Development (OP1) {ᴹ√KHYAD >>} ᴹ√SYAD had the derivative ᴹQ. hyarna “compact, compressed” (PE19/45). Similar forms appeared in the second version of Tengwesta Qenderinwa (TQ2) and Outline of Phonology (OP2) from the early 1950s (PE18/101; PE19/92). However, in between these two sets of documents, in the Quenya Verbal System of the 1940s, Tolkien gave ᴹ√SYAD the gloss “cleave through” as the basis for the verb ᴹQ. hyar- “cleave” (PE22/102).
The other element Sanga- of the name Sangahyando was derived from the roots √THAG or ᴹ√STAG, which Tolkien only ever gave glosses like “(com)press”. Thus I think it is best to assume the glosses in TQ1, OP1, TQ2 and OP2 were aberrations, and that √SYAD generally meant “cleave”.