-il-ilesuffix

The English representatives of Latin -ilis and -īlis, forming adjectives, sometimes also substantives, as in fossilis fossil, civīlis civil; agilis agile, juvenīlis juvenile. These suffixes are in origin the same, viz. -lis with connective -i-, which with -i- stems as cīvi-s, hosti-s, and in some other words, gave -īlis. In Old French, the latter came down as -il, e.g. Aprīl-em, Avril, April; the former regularly lost the post-tonic ĭ, and became -le (for -l'), as in humil-em humble, habil-em able, fragil-em, fraisle, fraile, frêle frail, gracil-em, graisle, gresle, grêle. Latin words adapted in Old French at an early date took the ending -il masculine, -ile feminine, e.g. civil, civile; later words have the ending -ile only, as agile, facile, fossile. Very few of these words in English have the suffix in -il, e.g. civil, fossil, utensil; in the 17th cent. we find occasionally -il for Latin -ilis, e.g. difficil, docil, fertil, steril; but modern usage in English as in French has levelled -ilis and -īlis under the common form -ile. In Walker's Pronouncing Dictionary -ile from -īlis is pronounced /aɪl/, and -ile from -ilis as (-il); but the more recent tendency is to extend /aɪl/, with some exceptions, to all the words.

2. Following classical Latin and medieval Latin ordinal numerals of the type quartīlis, quintīlis, sextīlis (English quartile adj. and n., etc.), -ile /-aɪl/ is used in Statistics to form substantives denoting (a) those values of a variate that divide a population into the indicated number of groups, equal in size, and (b) the groups themselves; so octile and quartile (1879), decile (1882), percentile (1885).