The term "phonaesthesia" or also known as "sound symbolism" is defined as the phenomenon of particular sound clusters or sequences which become associated with a particular meaning.
Unlike onomatopoeia, a word doesn't resemble something what sounds like (in spoken language) or what looks like (in signed language). Rather, it seems that language evolves in some way out of synesthesia, connecting human senses of language with language symbols.
The term "phonaesthesia" or "sound symbolism" doesn't only refer to speech (aural-vocal modality) but also includes the visual-spatial modality of language. Examples of English symbols and ASL symbols will be described below.
A group of English words like snort, sneeze, sneer, snarl, sniff, and snob is associated with nose. The "gl" in words such as glass, glare, glow, glisten, glitter, glimmer, and glaze suggests shininess. The words starting with "f" such as fly, flee, flow, fluid, and flicker suggest lightness and quickness. (Crabtree and Powers). There are more but a few of these give you the idea.
In signed language, specifically American Sign Language (ASL), has the version of phonaesthesia. The handshape "8" (either open or closed) is found in a group of the ASL signs: HATE, SICK, SICK-OF, MEDICINE, AWFUL, TOUCH, and FEEL. These signs suggest "health-feeling-ness".
The handshape "8" has other symbolism that is suggestive of emptiness such as EMPTY, NAKED, NUDE, SPACE-EMPTY, AVAILABLE, NOT+AVAILABLE, BALD, HAND-OFF.
In addition, a location-based phonaesthesia of the signed words such as GOOD-RIDDANCE-OF, NOSEY, PURPOSEFULLY-REVENGE, LEARN-LESSON-BIG-TIME (or LEARN-NOT-DO-AGAIN) suggests "curse-ness" or along the line. It's difficult to find a term to define something that lacks a word for a concept in either languages, English or ASL. But, that gives you the idea what phonaesthesia is about.
Another thing, vowels (vowel height) along with consonants in English words reflect perception of size. The vowels in these words "teeny", "small", and "tiny" feel small. And, the words "large", "big", "huge", and "enormous" feel big.
In parallel, the ASL signs SMALL, SHORT-HEIGHT, the mouth morpheme "oo", SUMMARY, and LESS are signed smaller or convergent.
The location-based phonathesia of the signs is horizontal in DEAD while the signs LIFE/LIVE and INSPIRE are upward or vertical.
M. Crabtree and J. Powers. "Arbitrariness in Language." Linguistics of American Sign Language, ed. by C. Valli and C. Lucas. Gallaudet Univ. Press, 2000.
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