neither upwards nor downwards

न मुखेनोद्गिरत्यूर्ध्वं हृदयान्न नयत्यधः।
जरयत्यन्तरे साधुर्दोषं विषमिवेश्वरः॥
सूक्तिमुक्तावली भगदत्त जल्हणकृतम्॥
na mukhenodgiratyūrdhvaṁ hṛdayānna nayatyadhaḥ|
jarayatyantare sādhurdoṣaṁ viṣamiveśvaraḥ||
sūktimuktāvalī bhagadatta jalhaṇakṛtam||

The noble and virtuous people never permit evil things arising within them or directed at them to come up beyond their throats, nor do they permit them to flow down beneath their hearts..
It is just like the action of Shiva in keeping the evil venom Kalakootam in his throat without permitting it either to go up or down.

The idea is that when a really noble person discovers an evil thought arising withing him, he just restricts it withing himself and kills it and would never speak out or take it to his heart.
When he receives bad words, thoughts or is affected by bad deeds of others too, he never abuses others verbally, nor does he takes it to heart.

To save the universe from destruction Shiva had to swallow the poison Kalakootam which arose from the churning of the milky ocean..
In spite of all possible dangers Shiva did not spit out the venom.. He did not gulp the poison beyond the neck too, so that it did not go below the heart and burn out the abdomen which housed the entire universe.. This analogy is brought in to highlight the conduct of good people..
Regards