From bottle +‎ neck.

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bottleneck (plural bottlenecks)

1. The narrow portion that forms the pouring spout of a bottle; the neck of a bottle.
   1. (music) A portion of a bottleneck placed on the finger and used as a guitar slide.
2. (figurative) In traffic, any narrowing of the road, especially resulting in a delay.

   Synonym: chokepoint

   • 1962 October, “Talking of Trains: The collisions at Connington”, in Modern Railways, page 232:

     "Permissive" working allows more than one train to be in a block section at one time but trains must be run at low speed in order to stop on sight behind the train in front. Such working is often authorised to allow freight trains to "bunch" together to await a path through a bottleneck instead of being strung out over several block sections, as would be necessary if absolute working were in force.

   • 2020 December 2, Andy Coward, “New-look Reading is willing and able”, in Rail, pages 58, 59:

     At one time, Reading was regarded as a notorious bottleneck in the rail network, almost incapable of managing the number of services that needed to pass through the station each day. [...]
     A road widening scheme at Cow Lane has also allowed two-way traffic to run beneath the railway for the first time, helping to remove a notorious road traffic bottleneck in the town.

3. (by extension) The part of a process that is too slow or cumbersome.

   It is easy to create entries; processing the paperwork is the bottleneck.

   The bottleneck in this computer program is the inefficient sorting process; we should replace it with a faster one.

bottleneck (third-person singular simple present bottlenecks, present participle bottlenecking, simple past and past participle bottlenecked)

1. (transitive) To slow by causing a bottleneck.

   The merge bottlenecked the traffic every morning.

2. (intransitive) To form a bottleneck.

   The traffic bottlenecked at the merge every morning.

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• “bottleneck”, in OneLook Dictionary Search.