soft – мягкий, слабый, добрый, тихий, см. «sofa», «sob»

soft (adj.) (староанглийское «softe, sefte» - мягкий, удобный, спокойный); из западногерманского *samftijaz – ровный, гладкий, спокойный, мягкий; из корня *som- подходящий, соглашающийся
Old English softe, earlier sefte, "gentle, mild-natured; easeful, comfortable, calm, undisturbed; luxurious," from West Germanic *samfti, from Proto-Germanic *samftijaz "level, even, smooth, gentle, soft" (cognates: Old Saxon safti, Old High German semfti, German sanft; and from a variant form with -ch- for -f-, Middle Dutch sachte, Dutch zacht, German sacht), from root *som- "fitting, agreeable."

From c. 1200 of material things, "not stiff, not coarse, fine; yielding to weight." From late 14c. of wind, rain, etc. Of sounds, "quiet, not loud," from early 13c. Of words, "mild, restrained; courteous" mid-14c. From late 14c. as "indulgent," also "physically feeble; easily overcome, lacking manly courage." From 1755 of water ("relatively free from mineral salts"), from 1789 of coal. Meaning "foolish, simple, silly" is attested from 1620s; earlier "easily moved or swayed; soft-hearted, sympathetic; docile" (early 13c.). In reference to drinks, "non-alcoholic" from 1880. As an adverb, Old English softe "gently;" late 13c. as "quietly." As an interjection from 1540s.

1675:
SOFT (soft, Sax., saft, Belg., souft, Teut.) – податливый, покладистый, глупый

1826:
SOFT, a. – нежный, мягкий, простой, тихий; S. “soft”; T. “saafte, sacht”; B. “zagt”; D. “sagte”; G. “sakta”; т.е. переход в связи с путаницей «ſ» и «f».

Т.е. это слово «спать», «сопать», «сопеть», замена «п» - «ph» - «f»; СПТ – SPT – SFT – SF;