ערב = ערף Malachi 3:4

[Isaac Fried]

It essentially means
'limp, slump, soft, relaxed, spreadable, mixable, pulverized. Hence, pleasant, agreeable, not strict. not tense, not anxious.'
The root is certainly related to רב RAB, 'many', and to רפה RPH, 'loose, detached', and therefore something to intermingle with, as here: Prov. 14:10
לֵב יוֹדֵעַ מָרַּת נַפְשׁוֹ וּבְשִׂמְחָתוֹ לֹא יִתְעָרַב זָר
KJV: "The heart knoweth his own bitterness; and a stranger doth not intermeddle with his joy"
NIV: "Each heart knows its own bitterness, and no one else can share its joy"
Here in the sense of 'decline': Isaiah 24:11
עָרְבָה כָּל שִׂמְחָה גָּלָה מְשׂוֹשׂ הָאָרֶץ
KJV: "all joy is darkened, the mirth of the land is gone"
NIV: "all joy turns to gloom, all joyful sounds are banished from the earth."
I think that עָרְבָה here is not 'gloom', but: decrease, or wane and fade gradually.
Here it is in the sense of 'soft' and hence 'pleasant': Songs 2:14
יוֹנָתִי בְּחַגְוֵי הַסֶּלַע בְּסֵתֶר הַמַּדְרֵגָה הַרְאִינִי אֶת מַרְאַיִךְ הַשְׁמִיעִנִי אֶת קוֹלֵךְ כִּי קוֹלֵךְ עָרֵב וּמַרְאֵיךְ נָאוֶה
NIV: "My dove in the clefts of the rock, in the hiding places on the mountainside, show me your face, let me hear your voice; for your voice is sweet, and your face is lovely"
Also here: Malachi 3:4
וְעָרְבָה לַיהוה מִנְחַת יְהוּדָה וִירוּשָׁלִָם כִּימֵי עוֹלָם וּכְשָׁנִים קַדְמֹנִיֹּת
KJV: "Then shall the offering of Judah and Jerusalem be pleasant unto the Lord, as in the days of old, and as in former years"
And here: Jer. 31:26
עַל זֹאת הֱקִיצֹתִי וָאֶרְאֶה וּשְׁנָתִי עָרְבָה לִּי
NIV: " At this I awoke and looked around. My sleep had been pleasant to me."

The English Etymology dictionary says that "please" originates in the Latin "placare" to soothe, to quiet, English, "placate", to smooth, to calm down. I think, further related to "flake", and possibly "flog", beat down. Also to the German "flach", flat.

עֶרֶב, 'evening', is the time the sun eases down into the sea.
עַרָבָה is possibly a tree of drooping branches (willow?).

See Judges 19:9
הִנֵּה נָא רָפָה הַיּוֹם לַעֲרוֹב
KJV: "Behold, now the day draweth toward evening"

Isaac Fried, Boston University

[Isaac Fried]

Related to ערב - ערף is ארב, 'crouch, hide, lie low, lie in wait', as in
Deut. 19:11
וְכִי יִהְיֶה אִישׁ שֹׂנֵא לְרֵעֵהוּ וְאָרַב לוֹ וְקָם עָלָיו וְהִכָּהוּ נֶפֶשׁ וָמֵת וְנָס
NIV: "But if out of hate someone lies in wait, assaults and kills a neighbor, and then flees"
or Job 31:9
אִם נִפְתָּה לִבִּי עַל אִשָּׁה וְעַל פֶּתַח רֵעִי אָרָבְתִּי
NIV: "If my heart has been enticed by a woman, or if I have lurked at my neighbor’s door"

Hence the אֲרֻבָּה ARUBAH, 'spill gate, pour down gutter', as in Gen. 7:11-12
בִּשְׁנַת שֵׁשׁ מֵאוֹת שָׁנָה לְחַיֵּי נֹחַ בַּחֹדֶשׁ הַשֵּׁנִי בְּשִׁבְעָה עָשָׂר יוֹם לַחֹדֶשׁ בַּיּוֹם הַזֶּה נִבְקְעוּ כָּל מַעְיְנֹת תְּהוֹם רַבָּה וַאֲרֻבֹּת הַשָּׁמַיִם נִפְתָּחוּ וַיְהִי הַגֶּשֶׁם עַל-הָאָרֶץ
NIV: " In the six hundredth year of Noah’s life, on the seventeenth day of the second month—on that day all the springs of the great deep burst forth, and the floodgates of the heavens were opened. 12 And rain fell on the earth"

Isaac Fried, Boston University