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The answer is lummox. I can't believe that you guys didn't get this one. The word is right there in the list of synonyms. I just knew that as soon as someone bit shifted the hexadecimal the suffix would appear and give it away... there are only 4 or 5 words that end with 'MOX' in the English language with (lc > 3)
[PRIMARY CLUE PATH]
Gematriacally 0x00584f4d4d554c46 them: ('F'+('L'<<8)+('U'<<16)+('M'<<24)+('M'<<32)+('O'<<40)+('X'<<48) == "FLUMMOX"
crowning the luminous : LUM
oscillating : back and forth or left to right
by the fifth power of two : 32
oscillating 0x00584f4d4d554c46 by the fifth power of two : (0x00584f4d4d554c46 >>= 32) == 0x0000000000584f4d in ASCII == "MOX"
conjoined : lum mox
but will always be nincompoops : lummox
[ALTERNATE PREFIX CLUE PATH]
Gematriacally luminous : 0x53554f4e494d4c4c
oscillating 0x53554f4e494d4c4c by the fifth power of two : (0x53554f4e494d4c4c <<= 32) == 0x494d554c00000000 in ASCII == "LUMI"
crowning the luminous : "LUM"