Polysyllabic adjectives with double 不
Oxford gives three usages for this construct:


  不...不...
  
  bù...bù...
  
  ① [used to make an emphatic negative form of two words identical or similar in meaning]
  
  不骄不躁 
  not conceited or rash, free from arrogance and rashness 
  
  不慌不忙 
  unhurried, calm, leisurely 
  
  不知不觉 
  unawares 
  
  ② [used with two words of opposite meanings to indicate an intermediate state]
  
  不大不小 
  neither too big nor too small, just right 
  
  不冷不热 
  neither cold nor hot 
  
  不死不活 
  neither dead nor alive, half-dead 
  
  ③ [the first 不 is the condition of the second 不]
  
  不见不散 
  We won't leave until we meet 


Collins also gives three, pretty identical, definitions:


  不 ... 不 ...
  
  (bù ... bù ... )
  
  1 (表示强式否定)
  
  " ⇒ 不知不觉" (bù zhī bù jué) "unwittingly"
  
  " ⇒ 不慌不忙" (bù huāng bù máng) "unhurriedly"
  
  " ⇒ 不折不扣" (bù zhé bù kòu) "one hundred percent"
  
  2(= 既不 ... 也不 ... ) "neither ... nor ... "
  
  " ⇒ 不亏不盈" (bù kuī bù yíng) "neither too little nor too much"
  
  " ⇒ 不大不小" (bù dà bù xiǎo) "just the right size"
  
  " ⇒ 不死不活" (bù sǐ bù huó) "half dead"
  
  3(= 如果不 ... 就不 ... ) "unless ... then not"
  
  " ⇒ 不破不立" (bù pò bù lì) "without destruction there can be no construction"
  
  " ⇒ 不见不散。" (Bù jiàn bù sàn.) "See you there."