The hiragana syllable く(ku). Its equivalent in katakana is ク(ku). It is the eighth syllable in the gojūon order; its position is か行う段(ka-gyō u-dan, “row ka, section u”).
Originally a compound of the 連体形(rentaikei) of verbs and adjectives and 所(aku); 上代特殊仮名遣(jōdai tokushu kanazukai) contractions of vowel clusters caused it to be reanalyzed as the suffixes:
く(ku) following
未然形(mizenkei) of 四段活用(yodan katsuyō) and ラ行変格活用(ra-gyō henkaku katsuyō) verbs