thick

The English adjective 'thick' is a word of remarkable etymological stability, having maintained its core meaning of density and breadth across three millennia of continuous use.‍​‍​‌​‍​‍​‌​‍​‍​‌​‌​‍​‍​‌​‌​‍​‌​‌​‍​‌​‌​‌​‌​‍​‍​‍​‌​‍​‍​‌​‌​‌​‌ It descends from Old English 'þicce' (spelled with the thorn character 'þ' representing the 'th' sound), meaning 'thick,' 'dense,' 'viscous,' and 'closely packed,' from Proto-Germanic *þikuz, from PIE *tégus meaning 'thick' or 'fat.'

The Proto-Germanic cognates form a tight, consistent family. German 'dick' means 'thick,' 'fat,' or 'swollen.' Dutch 'dik' carries the same range. Old Norse 'þykkr' meant 'thick,' and modern Icelandic 'þykkur' preserves the form almost unchanged from the medieval language. The consonant shift from PIE *t to Germanic *þ (th) is the hallmark of Grimm's Law, the systematic sound change that distinguishes Germanic from the other Indo-European branches.

In Old English, 'þicce' emphasized density and closeness more than girth. A 'thick' forest was one where the trees grew close together; 'thick' darkness was impenetrable; 'thick' speech was indistinct, as if the words were crowded together. The sense of physical girth (a thick plank, a thick wall) was present but secondary. Modern English has reversed this emphasis: physical breadth is now the primary meaning, with density a strong secondary sense.

The expression 'through thick and thin' is one of the oldest surviving English idioms. It appears in Chaucer's Canterbury Tales (c. 1387) in the Reeve's Tale: 'thurgh thikke and thurgh thenne.' The 'thick' and 'thin' refer to types of woodland — 'thick' being dense forest and 'thin' being sparse, open woodland. To ride through thick and thin was to cross all types of terrain regardless of difficulty. The phrase had already become a metaphor by Chaucer's time.

'Thicket' (a dense growth of bushes or trees) derives directly from 'thick' with the diminutive or collective suffix '-et,' and is attested from Old English 'þiccet.' This is one of the oldest compounds in the language and preserves the original density-focused meaning perfectly.

The figurative uses of 'thick' are extensive and vivid. 'Thick-skinned' (insensitive to criticism) dates from the sixteenth century and draws on the literal observation that some animals have hide too thick to be easily pierced. Its opposite, 'thin-skinned' (oversensitive), uses the same metaphor in reverse. 'Thick-headed' (stupid) dates from the seventeenth century, with the idea that a thick skull leaves less room for brains. 'Thick as thieves' (very close, intimately associated) dates from the early nineteenth century, where 'thick' means closely bound.

In British slang, 'thick' has meant 'stupid' since at least the nineteenth century, probably from 'thick-headed.' This usage remains common in British and Irish English but is less familiar in American English, where 'dense' fills the same slang role — a near-synonym that exploits the same metaphorical logic.

The phrase 'in the thick of it' (in the most active or crowded part) dates from the seventeenth century and originally described the densest part of a battle, where fighting was most intense. Kipling's 'The Thin Red Line' plays against this: where 'thick' implied a massed, dense formation, the 'thin' red line of British soldiers at Balaclava was dangerously sparse.

'Thick' as a noun — 'the thick of night,' 'the thick of battle' — is an old usage, attested from Old English. The word functions as a substantive meaning 'the densest, most intense part,' a grammatical flexibility that has been available to English speakers for over a millennium.

The word's phonological shape has been stable. The initial 'th' (/θ/) has been present since Proto-Germanic, and the short vowel and final velar stop have changed little. The Old English 'þicce' had a geminate (doubled) consonant, reflected in the Middle English spelling 'thikke,' but the pronunciation simplified to a single /k/ by the Early Modern period while the meaning remained essentially unchanged. Few English adjectives can claim such continuity between their Old English and Modern English forms.