Etymology
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excrescence (n.)
early 15c., "action of growing out," from Latin excrescentia (plural) "abnormal growths," from excrescentem (nominative excrescens), present participle of excrescere "grow out, grow up," from ex "out" (see ex-) + crescere "to grow" (from PIE root *ker- (2) "to grow"). Meaning "that which grows out abnormally" (on a living thing) is from 1570s (excrescency in this sense is 1540s).
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*ker- (2)

Proto-Indo-European root meaning "to grow."

It forms all or part of: accretion; accrue; cereal; Ceres; concrete; create; creation; creature; Creole; crescendo; crescent; crew (n.) "group of soldiers;" croissant; cru; decrease; Dioscuri; excrescence; excrescent; griot; increase; Kore; procerity; procreate; procreation; recreate; recreation; recruit; sincere.

It is the hypothetical source of/evidence for its existence is provided by: Greek kouros "boy," korē "girl;" Latin crescere "come forth, spring up, grow, thrive, swell," Ceres, goddess of agriculture, creare "to bring forth, create, produce;" Armenian serem "bring forth," serim "be born."

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knurl (n.)
"hard excrescence," c. 1600, probably a diminutive of Middle English knor "knot" (c. 1400), related to gnarled, from Proto-Germanic *knur- (source also of German knorren "a knotty excrescence," Dutch knor "knob," Swedish dialectal knurr "hard swelling"). Related: Knurly.
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outgrowth (n.)

"that which grows from or out of anything," especially an excrescence from the surface of a plant, 1801, from out- + growth. The figurative sense, in reference to inanimate things, "a development or result from a cause or beginning or from some earlier state" is by 1828.

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thymus (n.)
gland near the base of the neck, 1690s, Modern Latin, from Greek thymos "a warty excrescence," used of the gland by Galen, literally "thyme," probably so called because of a fancied resemblance to a bud of thyme (see thyme). Related: Thymic.
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sarcoma (n.)

1650s, "fleshy excrescence," Medical Latin, from Latinized form of Greek sarkoma "fleshy substance" (Galen), from sarkoun "to produce flesh, grow fleshy," from sarx (genitive sarkos) "flesh" (see sarcasm) + -oma. Meaning "harmful tumor of the connective tissue," more or less malignant, is from 1804 (Abernethy). Related: Sarcomatous.

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gall (n.3)
"excrescence on a plant caused by the deposit of insect eggs," especially on an oak leaf, late 14c., from Latin galla "oak-gall," which is of uncertain origin. They were harvested for use in medicines, inks, dyes.
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burl (n.)
mid-15c., "small knot in cloth or thread," from Old French bourle "tuft of wool," which perhaps is related to the root of bur, or from Vulgar Latin *burrula "small flock of wool," from Late Latin burra "wool," a word of unknown origin. In American English also "a knot or excrescence on a walnut or other tree" (1868).
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baroque (adj.)

"style of architecture and decoration which prevailed in Europe late 17c. through much of 18c., later derided as clumsy in form and extravagant in contorted ornamentation," 1765, from French baroque "irregular" (15c.), said to be from Portuguese barroco "imperfect pearl," which is of uncertain origin, perhaps related to Spanish berruca "a wart."

This style in decorations got the epithet of Barroque taste, derived from a word signifying pearls and teeth of unequal size. [Fuseli's translation of Winkelmann, 1765]

The Spanish word is perhaps from Latin verruca "a steep place, a height," hence "a wart," also "an excrescence on a precious stone" (see verruca). But Klein suggests the name may be from Italian painter Federico Barocci (1528-1612), whose work influenced the style. How to tell baroque from rococo, according to Fowler: "The characteristics of baroque are grandeur, pomposity, and weight; those of rococo are inconsequence, grace, and lightness." But the two terms have been used without distinction for styles featuring odd and excessive ornamentation.

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oak (n.)

"tree or shrub of the genus Quercus," Middle English oke, from Old English ac "oak tree" and in part from cognate Old Norse eik, both from Proto-Germanic *aiks (source also of Old Saxon and Old Frisian ek, Middle Dutch eike, Dutch eik, Old High German eih, German Eiche, Swedish ek, Danish eg), a word of uncertain origin with no certain cognates outside Germanic.

The usual Indo-European base for "oak" (*deru-) has become Modern English tree (n.). In Greek and Celtic, meanwhile, words for "oak" are from the Indo-European root for "tree." All this probably reflects the importance of the oak, the monarch of the forest, to ancient Indo-Europeans. Likewise, as there were no oaks in Iceland, the Old Norse word eik came to be used by the viking settlers there for "tree" in general.

In English the word is used in Biblical translations to render Hebrew elah (probably usually "terebinth tree") and four other words. The form in Middle English was very uncertain (oc, oek, hokke, ake, eoke, aike, hock, etc.). Oak-gall "excrescence produced by an oak tree in reaction to insects," used in making ink, is by 1712.

                            Jove's own tree,
That holds the woods in awful sovereignty,
Requires a depth of lodging in the ground ;
High as his topmost boughs to heaven ascend,
So low his roots to hell's dominion tend.
[Dryden, translating Virgil]
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