What does Hippocrates say about phlegm?

Answer 1

Phlegm is a sticky material from your mucous membranes in your repiratory system. When you have a cold it might be yellowish brown. If there's an infection, it might be greenish brown. Normal phlegm is usually clear.
I presume you mean: "Phlegm"?

Phlegm is a liquid secreted by the mucous membranes.

The definition is limited to the mucus produced by the respiratory system and particularly that which is expelled by coughing("Sputum"). Phlegm in essence is a water based gel consisting of numerous immuno-type substances. Its composition varies depending on a number of factors, such as climate & level of health etc. Its colour can vary from clear to pale or dark yellow & green, from light to dark brown.

Basically, it is a vehicle through which the body attempts to flush out foreign invaders from the respiratory system.
'phlegm' is an English word, meaning spit, saliva.
It is also used to indicate 'unexcitability, steadiness, boringness', and is used in the adjective 'phlegmatic', meaning 'unexcitable, steady, boring'