ぞくぞく

From reduplication of 簇(ぞく) (zoku),[1][2] itself from Middle Chinese 簇 (MC t͡sʰuk̚, “crowded, bunched, gathered”).

• (Tokyo) ぞくぞく [zòkúzókú] (Heiban – [0])[1]
• IPA^(key): [d͡zo̞kɯ̟ᵝzo̞kɯ̟ᵝ]

ぞくぞく • (zokuzoku) ^†-tari, adnominal ぞくぞくたる (zokuzoku taru), adverbial ぞくぞくと (zokuzoku to) or ぞくぞくとして (zokuzoku to shite)

1. 簇簇: (rare reading) grouped together, bunched up

Used with the adverbial particle と (to).

More commonly read as そうそう (sōsō).[1][2]

From reduplication of ideophonic element ぞく (zoku), also found in ぞくっと (zokutto) of similar meaning.[1][2]

• (Tokyo) ぞ​くぞく [zóꜜkùzòkù] (Atamadaka – [1])[1]
• IPA^(key): [d͡zo̞kɯ̟ᵝzo̞kɯ̟ᵝ]

A surface analysis suggests ideophonic ぞ (zo) expressing a sense of "buzz" or "shiver", appended with ideophonic く (ku) possibly expressing a sudden change in state.

Likely cognate with the zo element in ぞっと (zotto, “shiveringly, shockedly, startledly”), ぞぞめく (zozomeku, “to move or behave in a creeping, buzzing, rustling, impatient, or uncomfortable manner”), ぞぞ髪(がみ) (zozogami, “hair standing on end due to fright or shock”).

ぞくぞく • (zokuzoku) 

1. shiveringly, with a shudder
   • 2018, Charlie N Holmberg; Fumiyo Harashima (tr.), Shinjitsu no majutsushi, translation of The Master Magician:

     指(ゆび)の裏(うら)側(がわ)で首(くび)筋(すじ)をなでられ、肩(かた)の下(した)にぞくぞくとした感(かん)覚(かく)が走(はし)る。「ありがとう」とエメリーは言(い)った。

     Yubi no uragawa de kubisuji o naderare, kata no shita ni zokuzoku to shita kankaku ga hashiru. “Arigatō” to Emerī wa itta.

     The back of his fingers caressing the nape of her neck, a sensation ran shivering down her shoulders. "Thank you," Emery said.

May be used with the adverbial particle と (to).

• ぞっと (zotto)
• ぞくっと (zokutto)

ぞくぞくする • (zokuzoku suru) suru, stem ぞくぞくし (zokuzoku shi), past ぞくぞくした (zokuzoku shita)

1. to shiver, to feel a chill, as from cold or fear

   体(からだ)がぞくぞくしている。

   Karada ga zokuzoku shite iru.

   My body is shivering.

2. to be vibrating as from joy or excitement
3. to shudder from fear, to have the creeps

Zokuzoku implies an ongoing shiver, whereas synonyms zokutto and zotto imply a one-time shiver.

• ぞっとする (zotto suru)
• ぞくっとする (zokutto suru)

From reduplication of 続(ぞく) (zoku, “to continue”),[1][2] itself from Middle Chinese 續 (MC zɨok̚).

• (Tokyo) ぞくぞく [zòkúzókú] (Heiban – [0])[1]
• (Tokyo) ぞ​くぞく [zóꜜkùzòkù] (Atamadaka – [1])[1]
• IPA^(key): [d͡zo̞kɯ̟ᵝzo̞kɯ̟ᵝ]

ぞくぞく • (zokuzoku) 

1. 続続, 続々: successively, one after another

Used with the adverbial particle と (to).