
knapp
scarce
Available only in a small, barely sufficient amount.
Die Zeit wird knapp, wir müssen uns beeilen.
Time is getting scarce; we need to hurry.
scarce / just under / close
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scarce
Available only in a small, barely sufficient amount.
Die Zeit wird knapp, wir müssen uns beeilen.
Time is getting scarce; we need to hurry.

just under
A little less than a certain amount, number, or time; almost.
Er ist knapp 18 Jahre alt und darf bald Auto fahren.
He is just under 18 years old and will soon be allowed to drive.

close
With only a very small difference or lead.
Es war ein knapper Sieg für die Gastmannschaft.
It was a close victory for the visiting team.

brief
Short and limited to the essentials; without unnecessary details.
Auf meine lange Frage gab sie nur eine knappe Antwort.
To my long question, she gave only a brief answer.

tight
Very close-fitting; often so much so that it only just fits or covers.
Das T-Shirt ist mir nach dem Waschen zu knapp geworden.
The T-shirt has become too tight for me after washing.
In the meaning 'barely sufficient', 'knapp' (scarce) is very often used with abstract resources such as 'Zeit' (time), 'Geld' (money), or 'Platz' (space). It usually appears with the verbs 'sein' (to be) or 'werden' (to become/get).
Wir müssen uns beeilen, die Zeit wird langsam knapp.
We need to hurry; time is slowly getting scarce.
When 'knapp' (just under) stands directly before a number or quantity, it means 'almost' or 'a little less than'. In this function, it is an adverb and is not declined.
Die Reise kostet knapp 500 Euro.
The trip costs just under 500 euros.
When referring to results in sports or competitions, 'knapp' (close) describes a very small difference. Typical combinations are 'ein knapper Sieg' (a close victory), 'eine knappe Niederlage' (a narrow defeat), or 'ein knappes Ergebnis' (a close result).
Die Heimmannschaft verlor das Spiel nur durch eine knappe Entscheidung.
The home team lost the game only because of a close decision.
In connection with communication, 'knapp' (brief) means 'short and concise'. People speak of 'eine knappe Antwort' (a brief answer), 'eine knappe Erklärung' (a brief explanation), or 'eine knappe Zusammenfassung' (a brief summary).
Er war in Eile und gab mir nur eine knappe Beschreibung des Problems.
He was in a hurry and gave me only a brief description of the problem.
The meaning 'too tight' or 'too short' is used almost exclusively for clothing and fabrics. It describes that a piece of clothing no longer fits properly.
Nach dem Urlaub war ihm seine Lieblingshose zu knapp.
After the vacation, his favorite pants were too tight for him.
almost
„fast“ generally indicates that something has almost been reached or is almost the case. „knapp“ is used especially with a number, amount, or time expression and emphasizes that the value is usually slightly below it. For an approximate number, „fast“ is the general choice, while „knapp“ marks the value more precisely.

Knapp hundert Gäste kamen zur Feier.
Just under a hundred guests came to the celebration.
Fast hundert Gäste kamen zur Feier.
Almost a hundred guests came to the celebration.